Both ATI and nVidia are fairly closely matched in terms of performance, hardware ability, technical ability, and driver quality. The notion that ATI drivers are of any less quality than the nVidia ones are utter bollocks because I have both GeForces and Radeons around here and I run into the same shenanagins on both sets of Deonators (or Forceware or whatever you call it now

) and Catalysts. Find the drivers that work best and stay there.
Presently my powerhouse is a Radeon 9700Pro. Its fast, its pretty, and it looks really good in games. No problems, no issues, nothing to suggest that I made a bad purchase.
That said, see what card is best for you. The FX 5700 and 5900 are the best of the bunch, the FX 5200 is a budget card and the FX5600 and FX5800 are to be avoided. The Radeon 9800Pro, or XT are top of the bunch right now, the 9600XT is an excellent buy, and so on.
Be aware that budget versions of all cards in the ATI brand are known as SE (so 9600SE for instance) and the budget cards in the nVidia brand are XT (in an obvious attempt to sow confusion since the ATI top level cards are labled XT).
Read the reviews and see where it lands you.
Certainly the ATI cards, as Sandwich mentions, loose less FPS for enabling FSAA or AF....while without they are virtually identical to the nVidia cards. The GeForce 6 brand is supposed to redress that problem as well....