If you have the money and are planning on playing any of the upcoming games that take advantage of DirectX 9 programmable shaders (Half-Life 2, etc), then the 9800 XT would be the best card.
If you are not going to be playing the latest FPS or flight simulator games, then the 9600 XT should be good enough. I think it benchmarks at about 70/80% of the 9800, but I didn't check that very closely, since the price disqualified the 9800 in my case.
I went with a Sapphire 9600 XT 256MB, since that was pretty much the same price as other brand's 128MB cards, and Sapphire mostly seemed to have a good reputation. The 256MB versions are clocked slightly slower than the 128MB cards, but for the difference, the extra memory will likely hold out longer as the requirements are raised in future games. Plus, you can always give overclocking a try (if you are not worried about the warrantee), but can not upgrade the video memory.
