choose between 9600gt, 8800gt, and 4850. 9800gtx actually underperforms or equals the 4850 in plenty of cases. whats your REAL budget
If you aren't on a budget, ignore any card from nVidia's *600 line. Go for *800 only. frankly, at this point, I'm still recommending 8800 GTs since everything since hasn't been THAT hot. After my OG 8800 GTS died, I just bought a GT as an interim card until the new hotness came out. . .and it's still not here.
Also note that while the ATI card might have better numbers, after years of dominance, developers are coding to nVidia cards, and I'd safely say they still have the advantage when it comes to most games.
And although I love Smooth Groove, I would not recommend SLI unless you literally have money to throw away.