I've been a big fan of my XFX 7900GT that I got for $300 a couple of months ago. You can get a factory overclocked model from EVGA or XFX for less than $300. The ATI alternative in a similar price range would be the X1800XT or the newly-released X1900GT. In the reviews that I have seen, the 7900GT has been able to beat these cards in most games (the only notable exception that I saw is Battlefield 2. For some reason, that game always swings in ATI's favor). Good luck with your purchase!
Iamfalcon