Most Vista problems aren't really Vista problems but stubborn users who can't adapt to or figure out how to disable certain security features new to Vista. I run all of the SF games on Vista, and have played a number of other games both new and old without any problems.
Discreet sound is still generally better to have than most on-board solutions. Some motherboards offer an on-board chip from Creative, which is fine, but stay away from the Realtek and other El Cheapo stuff.
I see no advantage to SLI when a single GTX 260 offers so much performance. SLI would not be a factor in my motherboard choice.
Even a single-card video solution today requires a good power supply. Don't skimp on it.
Only a 64-bit OS can address 4GB of memory. I run Vista 64 with no problems. You need 64-bit drivers for all of your hardware. XP cannot address a full 4GB of RAM, and the 64-bit version of XP is really Windows 2003 Server with the XP interface. I've used it, and it works well, but driver support for new hardware will be a major headache as vendors aren't supporting it anymore (support was never good to begin with).
You would be better off with one of the 216-core GTX 260 boards.