Vista does support SLI, it can run both 32-bit and 64-bit programs, but not at the same time. That is why there is a 32-bit and 64-bit version. It's not as hard to find drivers for Vista, as it was before.
The only reason to go Vista, is if you're looking to play future games in DX10 mode. Other than that, it's just unnecessary eye candy.
And you're into graphics design, correct? I've heard that Vista doesn't do good in that area, so XP is the one to go for.
Which version? You're looking to get 4GB of RAM, and you're into designing and making 3D graphics, so XP 64-bit is your best bet.