using awesome wm for a triple head setup, which i guess is a lot lighter than unity or whatever.
I ran the same thing but in reverse before, windows 7 with a couple of linux VMs, for making web stuff to run on linux servers it was painful.
oh, and is it's vaguely relevant i bought the bits for a grand total of £700 excluding screens/keyboard/mouse/optical drive.
And sure there are lighter ways to test different browsers, but I don't trust them, been caught out before by silly differences like font rendering, testing eflyers outlook express/outlook etc, in the multi-ie-in-one-app things before, now i have a one-keystroke switch between an environment i can trust 100% to be authentic, and is easy to move around too.
basically using vm's in this way is one of the things that let me sleep at night.