Without Vista, you won't get Direct X 10 anyway, if you have the graphics card that supports it or not.
SLI though is still an iffy thing for me to consider. Its true, more games are becoming able to run very efficently with new drivers, and new games of course, are capable, yet, its still hard to tell in a lot of cases, if you are getting the performance you are asking for from buying 2 identical cards.
It is not double the speed with 2 cards, which you might think would happen naturally.
There is something inside of me saying to go for SLI, but another part of me is saying, yeah but buy a 8600 now, and you can upgrade with another 8600 when you upgrade to Vista later on, as you'll probably have more cash then.
Though, on the other hand, in the time you take to get Vista, you may want to upgrade your card further since better cards will indeed be out then, and the 8800, etc would have dropped, and would become affordable, and you'd be stuck with having to replace the 8600.
So in light of that, I'd say go SLI with the 7600, since like Celegorm says, you won't be getting much of a perfomance boost without Direct X 10
Games for a long time will support both, no worries.