Once available in quantity, my next vid card will probably be a Radeon X800XL VIVO edition of some sort.
If I was going nVidia, I'd probably go single 6800GT instead of two 6600GT's, then upgrade if the SLI bug bit. There are more games than flightsim currently not supported, World of Warcraft, Chronicles of Riddick, Everquest 2, and others aren't on the list as well (though they may be compatible if nVidia ever tests them and codes them into new drivers). Due to nVidia's slow driver releases, plus their broken promises on the onboard video processor for most 6800 cards, and the way they're marketing SLI (not being clear to end-users that it isn't supported in all games), I'll be going back to ATI next time around. My Radeon 9700 was a good card.
For SLI to even be a thought with me, nVidia would have to commit to driver updates the way ATI has. Since the Radeon 8500 release, they've really cleaned up what once used to be miserable driver support, in quality as well as in frequency of driver updates. nVidia's most recent WHQL drivers are dated November 9, 2004. ATI has had at least two driver updates since then.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890