On a Pro, it smokes (both win7 and Ubuntu). You'd probably be able to run Ubuntu without issues on an Air (vm'd) as well, I think I usually only give that VM 2gigs and it runs fine. I haven't tried running Win7 vm'd with less than 4gigs so I can't say. Probably rough though.
your term "it smokes" that would mean fast or slow? sorry, but I am not american
Haha, sorry my bad, I should have been more careful on my choice of wording.