This is going to be a very long post, but I hope it's going to help you make up your mind and put it behind. A lot will depend on comfort factors ultimately, and in any case you're looking at new Windows no matter what:
I honestly don't know if it's an OEM. When I linked to the
P6740UK, that was the original one that I purchased. I was too new to the game to really put everything together myself, so I basically bought that, then ended up swapping out the graphics card and power supply, slotting in extra RAM, and eventually giving it a new fan, and I haven't upgraded it since. I suspect that makes it an OEM but, honestly, I had to look up the term - you'd know better than me.
The problem with replacing the computer wholesale is that this isn't a dedicated gaming rig. I work from home in an online business, which means I need the machine to be reliably accessible throughout the working week. I can swap out parts and iron out OS problems over an afternoon or a weekend and recoup part of my investment afterwards by selling replaced parts, but selling the computer outright to help fund a later upgrade would take too long. It's not a workable option.
That in mind, my current plan is to get this
core,
this mobo, and
this RAM, see how that runs as stage 1, then look into an SSD. Maybe
this.
That said, I'd forgotten about auction sites. You're right, they're a good option to keep an eye on, but I only really know eBay, so if you have any recommendations for sites to keep checking, I'd appreciate it.
The OEM vs box matter is important because OEM Windows licences don't allow you to replace your motherboard, and it's unlikely that an HP Pavilion would have anything else than an OEM system. Hence, if you want to replace your mobo + CPU, then I would buy full Windows 10 (not upgrade) and also buy an SSD drive to install it on. At which point all you were to retain from the old box would be RAM, PSU and fan, which is not worth extinguishing a Windows 7 OEM licence and end-of-lifing a decent office computer that still serves you well, despite its limitations.
I was in a similar situation this spring and decided to take the graphics card, soundcard, DVD drive and my best PSU for my new computer but otherwise leave the old computer still operational with its OEM Windows 7 licence intact.
Next, since it's an HP Pavilion, I suppose it's quite likely you don't have an installation DVD but sort of installation partition on your hard drive like on a laptop, am I correct? If yes, then that's another reason to leave that system and that hard drive alone.
Re: auction sites, unfortunately I live in Poland where we have this huge auction monopolist that only works here and in other Central-European countries, so E-bay and Amazon isn't even really used, forget less popular alternatives. I've got zero experience of them beyond buying Kindle e-books. Your best pick would be someone local and not in a different country, preferably a company rather than an individual if possible, etc., just to keep you safe.
To order things up a bit:
(1) Replacing the mobo in your current computer is not an option unless you also buy new Windows.
(2) If you don't buy new Windows, it doesn't make sense upgrading anything in your current computer. Case closed, and you're looking at a new computer when you put together the budget.
(3) If you do buy the new Windows, then for practical reasons and convenience it makes more sense to keep the old Windows 7 licence and installation alive and operational. This means installing the new Windows (10) on a new drive, which would preferably be an SSD.
(4) If you buy a new mobo, new CPU and new system drive (SSD), then you'll essentially be building a new computer — new as in new for you, though it may be used parts, of course — even though you could initially cannibalize the RAM, graphics, DVD drive, perhaps chassis as well, just to keep costs low initially, though for practical reasons and convenience, it could be better to not cannibalize anything.
(5) If you want
to keep costs low and cannibalize parts from your current computer, notably RAM, then it makes sense to buy a used i5 or i7 (any generation from 2 to 5). It would of course be even cheaper (or you would have more money to spend on other parts) if you also cannibalized your current HDD, but we've already discussed this: as long as you don't have a proper installation DVD for your Windows 7 but only an installation/restoration partition on your HDD, formatting that HDD pretty much kills your old Windows 7 for the old computer. Meaning your old computer realistically won't be coming back to life. So this cheap option basically means killing off your old computer, you might as well sell off the parts. This is a reasonable option if you're on a tight budget and don't need or want a functional older computer to fall back on just in case.
The upgrade order would be as follows if you decided to cannibalize (all presumed to be used parts):
1. Mobo + CPU. Preferably a combo deal because buying either one first and looking for a match would put pressure on you to buy fast, overpay or settle for something less than you'd like. If not a combo deal, then you'll need to make sure you have both CPU and mobo from different auctions, which basicallly means Buy Now rather than bidding (bidding can result in surprisingly high prices for old stuff because people don't like losing).
2/3. SSD.
2/3. GPU. Not a huge priority because your current card is strong enough that it makes sense to put your money in the CPU first.
4(? — situational). Another 8 GB RAM before it becomes as expensive as DDR2 is now.
5. Good chassis.
(6) For practicality, comfort, convenience etc. you may want to just buy a cheap new computer, Skylake or Kaby Lake generation, with DDR 4 and also avoid cannibalizing the hard drive. If the budget is an issue, really just it put it off a month or two instead. There would be several different options here, depending on the budget and your upgrading strategy for the future:
1. If you believe you're never going to be interested in overclocking, then you don't need a Z170/Z270 chipset, and if you also don't believe you're going to be interested in M.2 drives or multiple graphics cards, then you can buy a mobo on the cheap since all you need is one graphics card slot, two RAM banks, 2 to 4 SATA connectors and perhaps a couple of USB ports, and that's it. You can then put the remaining money in the CPU — locked, because you won't be overclocking.
Here things branch out a bit, since there are three different things to focus on: CPU, GPU, SSD:
– If you have an even older GFX card to use with your old computer (or don't mind putting an £10 card in), then you can take your old 6950 to the new computer and focus on other things first.
– Otherwise, if you buy a new card, it doesn't make sense to buy anything weaker than GeForce 1060 or Radeon R9 480 if you do buy a new card, so their price how much money you'll have left for the CPU and the SSD (and anything else you may want to buy).
– Remember you have to include the cost of Windows 10 box (costs like OEM, you have to assign it to 1 PC, but you can change the assigned PC every 3 months).
This most likely means buying Windows 10, mobo, CPU, RAM, 480/1060, splitting the rest between SSD and CPU.
If you want to buy a cheap starter CPU, even a Pentium G won't be bad for office applications (especially with limited multitasking), older games, and strategic games or other games that use/need no more than 2 cores and depend on the CPU clock a lot. In many situations the difference between a Pentium G and an i3 will not be noticeable, and in some situations (e.g. certain RTS games) the Pentium G may be better simply because it's clocked faster. Bottom line, don't buy an i3 as a placeholder CPU. Either buy a top i5 and stick with it or buy a Pentium G with the intention to upgrade later, but nothing in between unless you find an awfully good bargain (like £10 difference between Pentium G and i3).
If you can't decide between starting with a Pentium G vs topping out i5 from the get go, perhaps the comfort of having a 500GB SSD rather than having to fit inside some measly 230 GB could provide a way out of that dilemma.
Also, if you decide to buy factory-new parts, tryo to get Kaby Lake instead of Skylake. Mobos may be more expensive, but in terms of CPU bang for the buck it makes no sense buying Skylake in the light of Kaby Lake prices at the moment. Chances are Kaby Lake may go up a bit, I fear, so it might be a good idea to seize the opportunity while they're still relatively cheap.
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Bottom line, you're looking at either a fast and cheap platform (CPU+mobo) upgrade or new computer purchase, but always with the added cost of Windows 10. The platform upgrade can be cheap (e.g. a still powerful platform for 200 pounds but also the cost of Windows 10 at about 85), but it will effectively kill off your old Windows 7 licence (because of upgrading your old computer), while the new PC will keep your old computer operational (with its Windows 7 licence) but it won't be cheap because of so many new things to buy (several hundred pounds no matter what, seven hundred for a decent full box). Which option is better comes down to your subjective comfort (how much time, money, nerve etc. you have and want to spend).
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EDIT: Sorry, forgot to comment on your part picks specifically. If you want a locked CPU, get a 7400, 7500 or 760 plus a cheap mobo from a decent manufacturer, with even two RAM slots and not many PCI slots to keep things cheap, because since you're buying RAM anyway you may as well buy 16GB already (2x8 or even single stick, or actually a single 8GB stick to save some money). But if you don't mind the expense, it's good to have a Z chipset because you can use faster RAM then. However, 2133 vs 2400 doesn't make a difference, and even 2133 vs 3400 isn't night and day. Spending the money on the CPU and the GPU instead will give you more power.
