For your typical Vista installaton please refer to Microsoft's minimums and then add some, sure you can easily get XP into a 10Gb partition, but Vista has a much different instalation routine where it copies compressed files from the DVD then expands them on the same hard drive/partition, then it creates things like the swap file which in most cases is equal to the amount of RAM you have, it also creates the Hibernation file, again, same size as the amount of RAM you have in your machine, then there's the issue of System Restore reserved space... I am a long time WIndows user from 3.11 al the way to the unreleased Windows 7, Vista should have at least a 40Gb partition for installation, Gives the setup routine a lot of space for the junk it creates during instalation.
XP, you can get away with 5Gb.....just something about Vista. Besides, hard drives are large nowadays. If you want an honest useable Vista installation in Bootcamp on a Mac, go for atl east 40Gb, gives you some room for a few apps post setup. Personally, I'd do at least 60Gb for a really useful setup, allowing space for games and the aps you need.