I've read all the reviews and it clearly seems that the ati cards perform better than the nvidia cards, but I have been always been worried with ATI's driver support and compatibility problems.
I have always bought nvidia cards so therefore I'm biased, some google research shows:
searching: nvidia 470 problem
searching: ati 5870
searching: ati crossfire problem
searching: nvidia sli problem
I know that these results don't mean much, please vote which do you think is your favourite setup and explain why.