There are a lot of threads on this subject floating around. Since I'm such a swell guy, I'll sum up what I've learned (both from reading those and trying it myself) :

muti-monitor support in XP, from a performance standpoint, sucks. The most useful mode, dual-view, has a performance hit. If you run span, you have to have the same res monitors, which most often is probably not the case. Having more than one video card doesn't help, it's a problem with the windows rendering model.

multi-monitor support in Vista is better, and is supposed to fix this probelm. Whether it does or not is not entirely clear to me, but I'd say generally speaking it's better. However, overall vista comes with its own share of issues, that may offset any gains made by this improvement - or worse.

I'd suggest doing some searches for this info (it's spread about a lot different threads) and draw your own conclusions. That's just what I've surmised.