I'm still debating... I had a GF2 GTS that wasn't too great. It had decent graph capabilities for it's time, but would cause my computers to freeze up every so often. I now use an ATI Radeon 8500 which is great. I once thought I'd stick with ATI, which they seem to be on top with single card setups, but that SLI setup from nVidea is very intriging!
Really it depends what you want to play. ATi are much better for Half Life 2, whereas Nvidia are faster for Doom 3. When the engines for these games are used for others, you'll find that some give better performance on an ATi or Nvidia card. Personally, I would go for ATi. They are cheaper, faster, support Pci express to a better degree (more choice) and the top of the range x850 is immense.
Ummm, 99tollap, WTF do you mean more choices on PCIe????? NVidia has just as many models.. 5550, 5750, 6200, 6600, 6600GT, 6800, 6800GT, 6800Ultra, and Gigabyte 3D1... ATi is good, but I LOVE nVIDIA!! Go 6800Ultra or better yet, GigaByte 3D1!!!!!!!!!!!! 3D1 is a dual GPU on one card, dual 6600GT processors to be exact.