http://www.anandtech.com/video/showdoc.html?i=1863
that's about half life 2 and ati vs nvidia.
personally I would prefer ATI at the moment. the Radeon 9800 Pro 256 rocks everybody's boots off. with 256 mb memory on it it works like a charm on any game or util u wish to spin on your pc. for most games that 256 is ludacris. you don't need it. you could settle for the 128 card aswell tho some games will be taking advantage of the extra memory.
ATI seems to score better in just about any test performed. their chipset is simply more advanced tho things could change when the NV4x chipset from NVIDIA will be released. don't expect that chipset any time soon though. could take several months and in the meantime, ATI is working on a new chipset aswell, tho I wonder how fast they think they need to go.
It's like with CPU's. at the moment you won't see a lot of difference between a P4 3.2 ghz and a P4 2.5 ghz or something. CPU speed is no longer the bottleneck. same will go for graphics cards soon if you ask me. not sheer power but extra features and price will be more and more important, and, to some extend, new and improved features.
currently bottlenecks are more and more at datatransfer from memory to CPU and from HDD to CPU if you ask me. both CPU aswell as graphics cards are getting very sophisticated. ahwell... let's just see what the future brings
back on topic: go for an ATI Radeon 9800 or 9600, take the Pro version and you're set for the future. check the link I placed above and see what those cards do in a DX9 game. but also, check what they usually do in benchmarks, they simply rock. NVIDIA has been resting on their laurels for far too long, neglecting to listen to the gaming community and delivering a chipset which simply sucks. a GF Ti4600 will do nicely if you don't require the max, but I would never go for a GF ti5xxx card if I were you.