Even with the stock cooler you can most likely overclock the 7950 to easily keep up with or beat a stock 670. I've had my 7950 for about 2 months now, I normally keep it stock but when I want extra power I have no problem overclocking it past the levels of a stock 7970 while 100% stable.
It's a little game-specific. However, in the games where the 7950 is better, it tends to edge out the 670, whereas in the games where the 670 is better, it sometimes blows away the 7950. See here: http://www.anandtech.com/bench/Product/550?vs=598
You will probably have a little more luck OCing the 7950 (even reference) than the 670, so you can make back a little of the performance gap. However, as deemo13 said, you won't be disappointed with either card. I don't think it would be irrational to get the 7950 given your prices.
As to resolution, I'd think the 7950 might actually do a little better comparatively at high resolutions rather than low ones, since it has more VRAM (3GB, vs the 670's 2GB). But of course both cards will do better at lower resolutions.
From my own research I've gathered that the GTX 670 is the destroyer of cards...
However my budget is around £280
The cost of a new HD 7950 is £264 (good price I think) - only a reference card, no aftermarket cooler
Where as the GTX 670 is about £320 - only a reference card too...
(Plus the 3 game bundle is really tempting as I'd love to get my hands on DiRT ShowDown as I enjoyed DiRT 3.)
Is the ~£75 difference worth it? From my research it looks like I'd only be losing a few FPS, and I'm only going to be running a 24" @ 1920x1200, nothing monsterous.
well for that pricing difference the 7950 is worth it much more for performance per dollar, and don't forget that you can easily overclock the 7900 series cards well over there stock clocks. i'm running my 7950 at 1000MHZ which is something like a 20% overclock, which is reaching or surpassing the stock 680's in average performance. the only real flaws is it well use a lot more power and drivers are a little more likely to have problems.
I don't care about power, I've got a nice quality Corsair 650W, so will be ideal for me to go the for HD 7950 as I'm also saving myself a fair amount of money, and it's not like I'm going to be disappointed.
My 7950 runs a tad hotter than normal - about 35-40C idle and shoots up to around 65-75C under load - however my case is quite cramped and there's isn't much airflow at all. My main use for my computer is a HTPC/gaming computer.
@hammerhead2008 I think it's a good idea to go with the 7950. At that price IMO it's the best bang for the buck. IMO the 75 price premium is not worth the extra ~10% difference in performance.
Should I try and go for a card with an aftermarket cooler on it?
Or does the difference not actually make a difference
aftermarket coolers are always a good idea for cooler and more stable cards, but there not a must if you don't plan overclocking. the reference cards are just fine, but for 10 to 20 bucks more in some cases the custom cards are great.