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 results: 554.000
searching: ati 5870 results: 602.000
searching: ati crossfire problem results: 251.000
searching: nvidia sli problem results: 4.330.000
I know that these results don't mean much, please vote which do you think is your favourite setup and explain why.