IMO unless you want to play the new DX 10 games that will soon be coming along, there is absolutely no reason at all to use Vista. (And there also you could simply set up a dual boot for the games) XP 64 is a very capable OS that gives you all the benefits of 64 bit without any of the crap of Vista and I for one will not touch Vista for my work rig.

Oh and yeh DosBOX rocks, it lets you run almost any DOS based app you can think of and it emulates a VGA card and a SBlaster sound card so you can play all them old games as well. And its free !