SLI performance for gaming is wonderful. sadly i have only done this in windows, but last time i was on the Nvidia page reading over their HOWTO guide, they now have SLI support for the linux drivers. as i get as good if not better performance from my Nvidia card under linux then windows, i can only guess their SLI support will be comparable to that of windows.
if you are not playing games, or not doing major graphic rendering like CADs type work, there is little need or use for SLI at this point in time for most desktops.
[edit to add] I LOVE THE SPELL CHECK BUTTON