It all depends on what kind of gaming you are mainly wanting to do and with what features. I myself had the volt-moded 7800GT SLI and went to a singel X1900XTX. Loved the switch. My games are better looking and the performance is the same if not better in some titles. If you've got your mind set on Nvidia though, the 7900GT in SLI would probably be the better buy, especially if your really wanting to run dual GPU's. But if you want better IQ and stuff, then ATI is probably your best bet.
We really need a few more specifics before any of us can give you some good knowledge as to what would best serve you. Amount you're wanting to spend, settings you're wanting to have, games you're going to play most often, etc. Anything to narrow down the selections can help us, and you, out.