Depends on system as their are lots of programs even in XP compatibility mode wont run properly, you need the minimum spec for vista.
Dual core processor and 4 gig memory as vista takes up 40% system resources as a lot of people put XP on as well and dual boot.
Vista is a stop gap operating system while he gets Vienna ready for release as if you look at 1 98 and 2000 they were stopgap operating systems.
I will stick with my xp as i have XP 64 and 32 bit pro XP and 2 Spare XP homes and 1 pro, if you don't have XP get it of eBay as prices gone up as looking at it from January first next year.
Microsoft will have destroyed most stocks of XP apart from those that they cleared out, they want people to only be able buy Vista from Jan 1st not XP, so get an XP now before the prices skyrocket before Xmas.