I use Windows VISTA Ultimate, Mac OSX 10.47, and of course Windows XP Pro. Mac OS X is cool but I feel more at home with VISTA. Vista so far does not crash that much when i play games, download files, and run Remote desktop. The Mac OSX don't have too many games so I only use it to surf and listen to Itunes.
It recommends to have 512 mb of ram, but really a gig is necessary to operate it smoothly. I'd suggest two gigs of ram.