I'm in the process of doing this at the moment - albeit Gentoo, not Ubuntu.
It took me a while to find a LiveCD that would boot properly (Kanotix was the key there), and now I'm banging my head against WiFi issues as the RT2500 drivers are somewhat immature and buggy, but it's getting there.
SLi hasn't been an issue for me - the config that Xorg spat out first off configured itself as dual-head, but that's easily stripped out to a single head at least to get things up and running.
As for RAID - it should be supported, but I must be honest - I disabled it long ago so that a) Norton Ghost would work, and b) I could put Linux on the second physical drive and Windows on the first. Means I can get a bit of a power save by having the unused drive power down when in each OS.
What specific problems are you having?