If you're geeky about one thing, bwaarp, you're going to be geeky about many things and there are a lot of bike/tyre/suspension/etc. geeks on here.
I believe that my PSU is 550W. Currently my PC has a borrowed GeForce 8800, which seems to be a fairly large card and also requires a PCIe 6 pin power connector (or something like that), so I know my PC can provide space and additional power to a reasonable extent.
The Radeon 7770 seems a little beyond my budget (as is the GeForce 560), but the 6850 hits the £100 price point. Tom's Hardware seems to be ranking the Radeon 6850 and the GeForce 460 as equivalent but it looks like overclockers.co.uk at least is charging 20 quid more for the nVidia card than the AMD one.