SLI can't do multimonitor on Vista. There is a workaround to do it on XP, but it's a bit of a hack involving cards from 2 different manufacturers.

My machine is SLI ready. If I want to play a game that will benefit from SLI I just go into the nVidia CPL and turn it on. No reboot required. It's a pain but not really too bad. I'd probably get it again if I was building a new computer, though with 2 8800 GTs now instead of GTXs. (Lower power / heat, almost the same throughput.)

My 2 cents.