Building 1st PC...where to begin?

Right, i have been given the task of building the family a new PC, only problem being i have no idea where to start. I know the components i need and i know how to put it all together but aside from that im lost.

I have a budget of around £750-£800 and this is what im looking at:

Case - not sure what i will need, power supply? fans?
Processer - Intel P4 3.0 ghz, no idea when it comes to AMD, what would be the eqivilent?
Ram - 1024mb 400/533mhz [ which would be better, what is good RAM, what is to be avoided?]
Hard Drive - 120-160gig SATA? RAID?
Graphics card - looking at a geforce 6600GT
Motherboard - havent got a clue, any advice for a mid spec one, which wont hold back the ram, g-card etc?
CD/DVD drive - not too sure with this, but they cant vary in spec that widely.
OS - would i be able to use the one which came with this PC - WinXP home(one being replaced, still got all numbers + codes for it, all legit)

Am i forgeting anything?

Sorry to put it all to you guys, but i dont want to make a mess out of this, we also want to get this done ASAP.

General advice, prices, what to avoid etc would be greatly appriciated.

This PC will be used for gaming(Planetside, half life 2 etc, along with your usual MS word etc)

Also does anyone know any sites where you can sort of build yourself a PC online using a configurator tool then you can see what you need, and can keep a constant tally of the cost of it?

many thanks.

Case - Go with whatever looks good, but make sure to read the reviews first so that it's good quality. I'd personally recommend the Antec Super Lanboy, $78 from newegg (dunno what that is in £, sry)
Processor - Go with an Athlon 64 3000+ Venice. AMD processors are a lot better than Intel's for gaming, and they're cheaper too so go with that one =P
RAM - 1gb ddr400 is good. I would recommend something from OCZ, they tend to have good quality ram.
HDD - 160GB Maxtor SATA150 would probably be best. I don't know much about raid tho so ask someone else for advice on that part.
GPU - If you're buying a new system, at least get a 6800GT or a X800PRO. If you'll be oc'ing get the X800GTO2, b/c it can easily be flashed to X850XL (i think that was it)
Mobo - Epox Nforce4 Ultra. I don't know the model, but the company and chipset should be enough for you to find it.
CD/DVD - Go with something from Lite-on or some other company like that. Don't get Plextor, they're just the same quality but more expensive, you're just paying for the name with them.
OS - Either Win 2000 or XP. Since you already have XP, keep it.

Edit: Almost forgot power supply. IMO a Fortron Blue Storm 500W should be more than enough to support your comp and keep some room for upgradability.

If you want to go with AMD, then there are several good A64 options. But, be careful about the fanboy claims of "AMD processors are a lot better than Intel's for gaming". The number 1 factor in gaming is the vid card. I have an Intel rig that will beat a lot of AMD systems in gaming. Conversely, a lot of top of the line AMD rigs will beat mine. It all depends on how much you want to spend. As for budget, you can build a good Intel P-4 rig for about the same price as an good AMD A64 rig.

I will concentrate on Intel options since I know them better. Intel "generally" is better at video encoding and multi-tasking. Also, Intel chipsets are still the best and most stable on the planet. If you are building a system from scatch, I highly recommend going with socket LGA775, DDR2 RAM, and a PCI-E slot video card. This will futureproof the system for possible upgrades as you need 'em.

Before I make specific recommendations, will this be overclocked? If so, you might need to include some cooling stuff in the budget. Seems like 1 pound equals almost 2 US dollars or am I confused? So, looks like you should have enough cash for good stuff. Do you already have a monitor, keyboard, mouse, and speakers?

Motherboard: Abit AW8 or Asus P5WD2. CPU: 630 (3.0 gig) with 64-bit support and 2M cache (these bad boys overclock great, 4 gig is no problem with good cooling). RAM: OCZ, Corsair, Kingston, Crucial, or Mushkin brand DDR2 are all good. If you might want to go 2X1gig, then check out PQI Turbo DDR2-667 which is reasonable priced. Vid card: get the best PCI-E you can afford.

Unless super fast HDD speed is required, I would get one nice 160 gig SATA HDD like the Hitachi (Maxtor and Western Digital are good too). Sometimes RAID setups can be a hassle. Well, good luck.

big thanks for the replys, spent a few hours with a mate going thru some components and came up with this setup for this price:

that is about $1000 give or take one, 1£ = 0.57$

edit: i also wont be going down the O/C route, and i will be buying everything from www.scan.co.uk, hence sort codes.

is it worth getting a slightly worth MOBO or do i need something that powerfull?

say swop that mobo and get something like this one:
http://www.scan.co.uk/Products/ProductInfo.asp?WebProductID=209818

No MSI. Period. Gigabyte, Epox, ASUS, they're all good. Just please, no MSI. ALWAYS go with good motherboards/power supplies/memory. You're fine with the mobo you have now. As for power supply... I've never even heard of HiperPower. IMHO you should go with a Fortron 450W instead. It's shouldn't be much more expensive. The rest of it looks ok considering you won't be oc'ing.

Edit: The only Fortron on that site is the 500W Blue Storm, so... hmm....

Yeah, go for that Forton PSU.

Maybe a different motherboard? If you wish to OC, go with the DFI LanParty Ultra-D, if you want to do some OCing without that much trouble, as the DFI requires a lot of troubleshooting and tweaking for it to work at times, then the Epox nForce4 one would be a great choice.