[Cheesus]

Well the main issue with SLI is that some older games have issues with it. But then again, for older games 1 GTX should be more then plenty.

Personally, the only real flaw I see with getting 2 8800GTXs and wanting to future proof is that in the future, it'll be harder to upgrade.
Imagine you get an 9800GTX now... You'll be able to max out just about every game available with a small overclock. When that isn't enough, then you can just pick up another one, and get yourself right back in the game.

Also, since you're pretty much buying a whole new computer, it'd make sense to leave the 8800GTX where it is, leaving your current pc as a backup or secondary PC (or even selling it).