Honestly, I'm happy with Mac OS X 10.4. It can emulate pretty much anything. (Except for the DS) Since it has a Unix/Linux base, You have the whole open source and shareware community behind you, and it's pretty easy to get good software. And it also has the same stability as a webserver.
But if you already have Windows XP, then you should keep that, because it would cost too much to switch. But if you want to get new stuff, I would rccomend a Mac.
