well, all the graphics power will go into the following:
1. 3D desktop acceleration - the windows desktop right now is 2d, and is NOT hardware accelerated. That's why you see games run smoothly at 100fps @ 1280x1024, but the desktop and windows paint so pathetically sometimes.
2. Dual- or even quad-monitor desktops - with dual card SLI setup, you can already setup 4 unique monitor combination (ie. each monitor will display a different/unique screen). Games will benefit largely from this. Imagine this - Battlefield2 actually supports a Commander mode - imagine how cool it would be if one monitor showed the entire map and all your troops and their positions overlaid on the map, and can be used to display commands, and all the other monitors displayed different player cams - THIS WILL ROCK!
3. Multiple VIRTUAL desktops - something like what the
http://download.microsoft.com/...skmanPowertoySetup.exe free powertoy - I believe these desktops take up more video buffer (double/triple buffer "pages") and so if you have dual cards, you have more video RAM (256MB x 2). This should make switching between different virtual desktops more smoother.