Well, it's a long asked question, but I've been looking for a long time and I am going to by ATI. Now it just depends on how much you can spend and if you plan on overclocking it.
I want to buy the 256MB memory version board, but they are not cheap. I feal I will get enough performance out of a top line 128MB version, at a cost effective price.
You just need to do a little research and make some comparisons for yourself. Price and performance between ATI and Nvidia...I'll go with ATI this time.
Good luck and stay in touch. Let us know how your experience goes.