Bring your system to its knees.
I dont have the most extravagant system around:
Amd64 3000+
1.5 gigs of Ram
BFG GF 6600 GT Overclocked
But I was able to run OSX Tiger and Windows Vista ontop of XP Pro at the same time! 
I left an mp3 playing to see if it would get choppy, which it didnt.
