What I would do (and did do) is order the 6600GT (or 6800GT) for now, and go AGP. Save your $$$ because only games that are designed to work with SLI will actually use both video cards, which won't do you much good for another 6 months or so (when they drop in price anyway). I do know that nVidia and AMD are working on package sets and multi-purchase discount deals for the next fiscal year; the goal is to make twin-card arrangements as affordable as comparable ATI single-slot cards. You'll also see a lot of goodies designed for nforce4/SLI combinations, often offered as a single package purchase for around $600, possibly with pre-939 64's in the mix.
THat's why ATI's crying foul, even though the got the X-Box2 and all the Nintendo units. nVidia nabbed the PS3, and they're working on making it very difficult for ATI chipsets to perform as well with AMD components as their own by creating bonuses for combining nVidia and ATI, and Intel isn't interested in backing a specific card so ATI is left out in the cold.
Needless to say, an ATI on an nForce4+ motherboard may be a bottleneck in and of itself in the near future.
edit: Accidentally put PS2 where it now says PS3, had to fix it.