currently now ATI offeres the most powerful graphics cards , the 5870 is on par with the Gtx295( Nvidias best card and is dual core while the ati 5870 is single core) and the 5890 which is basically 2 5870s.
and crossfire works better then sli, but you dont need to get the top of the range, and ati offer you great cards for money, but right now the best is ATI 5890 but then again nvidia is releasing gt300 series which is a beast of a card.
if you are not sure , compare the the 2 cards from nvidia and ati and see the benchmarks, and also the price,
generally some ati cards would fall a bit behind some nvidia cards but they are also about 100~200 dollar cheaper etc
