Get the GTX 275!!!
If I were you I would get the XFX GTX 275 XXX Edition (Factory OC), thats the card that I ended up getting after months of researching on what card to get, and oh boy am I glad I did my reasearch, It runs like a dream!! I cannot wait to see that babe in SLI 😛
If you care about your games having the best visuals... then go for a Nvidia GPU, but if you think that speed is everything then your best off following the ATI fanboys...
ATI card do have faster frame rates (at times), but not enough to make a differance, well not enough for the human eye to even notice... so thats why most people tend to go for ATI cards, because they go a ants fart bit faster.
But for the lose of like 1 or 2 FPS slower on a monster of a game like Crysis the nvidia cards really excell on the graphics quallity, in allmost every game nvidia's graphics quallity beats ATI's
but yes, to answare the question... Get the GTX 275, you will not be disapointed!