With VMWare 5+ and its 3D acceleration XP can do very very well, except for anything that uses shaders. When I was playing with it I had an XP client running under an XP host (a Vista host is likely to be more demanding obviously) and I only tested 2D games, but with the acceleration turned on the XP client was running games like Tiberian Sun and Red Alert 2 about 50-75% of the speed of the same games on the XP host. I didn't test any 3D games but I read that they have a similar performance, excepting 3D features that are missing or broken.
Standard Windows Desktop programs such as IE (video streaming/flash) and Office etc etc weren't accelerated from what I could tell and bore much the same results as without it turned on - which, depending on your host machine of course, all runs at quite acceptable speeds.
If you're really into gaming, you'd be better off dual booting, but if Vista is serving you fine, there's only 1 or 2 games that you'd play in XP or you're simply testing Blitz games, then virtualising might be quite acceptable for you.
Standard Windows Desktop programs such as IE (video streaming/flash) and Office etc etc weren't accelerated from what I could tell and bore much the same results as without it turned on - which, depending on your host machine of course, all runs at quite acceptable speeds.
If you're really into gaming, you'd be better off dual booting, but if Vista is serving you fine, there's only 1 or 2 games that you'd play in XP or you're simply testing Blitz games, then virtualising might be quite acceptable for you.