How do two GTS's perform compared to a single GTX? I was under the impression that the GTX would beat them more often than not...
SLI and Vista 64 still seems to be 'beta' as it's only been implemented in the latest sets of drivers. The Vista 64 drivers have finally stabilized for me with a single 8800GTS, but they still leave a fair bit to be desired in terms of some minor glitches and performance issues.
Basically I think you're stuck waiting. Nvidia posts beta drivers on their site fairly frequently, so you could always try those (although I've always had bad luck with them - very unstable).