Hi all i am buying a pc to 5box on (spec below) and want to know what OS people are running?

vista seems a bit more demanding on a comp and if im 5boxing dont really want that! and is 64bit much more demanding also or not much in it?

Quad Q6600
4GB ram
500GB 7200rpm hd
8800GT 512mb
P5N-E SLI motherboard