Your whole idea is somewhat counter productive frankly.. You've got an old machine you want to breathe some more life into. Best suggestion is spend as little as possible on it. Save for a new system which you likely won't even need to overclock because it'll mop the floor with your current so much you won't need to worry about anything like that.
I still stick to my suggestion of a Cooler Master 212 cooler of some sort ensuring it has the brackets for 775, and go from there, because you can ALWAYS re-use that in the next build, as you'd realistically only EVER need to change it if you overclock on the next system (and for most, it's not 'required').
Spend the least amount on your current system, and again a reasonable/cheap ish cooler is most likely good enough for your next as well.
Especially if you plan to upgrade anyway.. Why spend so much?
+1. Just find the best bargain for a 775 cooler right now — and that's going to be cheap precisely because it can't fit any other ('higher') socket out of the box, i.e. without a special mounting kit that costs extra.
By cooler I mostly mean heatsink. It will most probably be cheaper to buy a heatsink with whatever fan the previous owner was using (not necessarily the original fan from the manufacturer, not like that's necessarily a bad thing), but some sellers prefer to keep their good fans and reuse them. So should you. So you could, for example, get a naked heatsink and then look for the best fan that will fit. (Absolutely not under 92mm and preferably not under 120mm). When buying such old stuff, the fan may very well be more expensive than the heatsink.
And the difference goes right in the piggy bank for your next rig. Incidentally, your next rig is likely to be Skylake or a newer 1151 Intel CPU, which is a bit more fragile and has otherwise changed construction a little compared to older 115- processors, which affects heating properties and in some cases even damages the CPU due to excessive pressure with standard manufacturer-supplied mounts (it was serious enough for Scythe to give people improved mounting kits free of charge). Hence, you'll be better off with a dedicated 1151 cooler when (if) you get to that slot.
Or, if you saved $80 but still wanted an upgrade for your existing config, you could very well buy a strong 771 Xeon that's easy to mod into 775 or sometimes comes already modded. Those 771 Xeons require a strong chipset like P45 (check compatibility online) and aren't completely safe to use; some damage to the mobo is quite possible; on the other hand, we're talking about an old mobo that would be cheap to replace or you could just accelerate the purchase of a new computer in the odd event the Xeon upgrade actually caused some damage after like 1 year of use or something, which some people have reported experiencing.
On the other hand, you could legitimately ask 'why bother' and skip the Xeon route. A Xeon could run seriously hot on your existing cooler if it isn't a strong one, and OC headroom could be small regardless. By contrast, your Quad could be a better overclocker with a strong cooler if you bought one (but you'd have to check, as there are no guarantees).
Then there's SSD if you don't have it already; at least for the system and some of your most frequently used applications, to speed up reading and writing, if — and only if — you intend to keep your existing system for a longer while. A 128GB SSD should be very inexpensive right now but would be of little use in a future rig.
Back to the original idea of just replacing the cooler and keeping it for a future upgrade (6 months? 1 year? 2 year?), it can still be a good idea if you do it to have some OC fun with the quad in the meanwhile.
This spring I ended up buying a Thermalright Ultra — once a somewhat expensive somewhat high-end cooler — for like $15 shipped, fan included. I was certainly tempted to delay replacing the entire rig just to have a couple months' fun with my old lapped Wolfdale C2D. I kinda I wish I actually did just that and spent money on something more useful than a computer upgrade (nice holidays or some continued education).
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