A good rule of thumb, estimate what you need and double it. 50GB is generally enough for most people. Note that a modern game can easily use 10GB+. Also, if you use ISO images instead of carrying the game disc, thats another 8GB, Windows needs a few gigs for XP, much more for Vista and Windows 7. Don't underestimate space you need.
Spare what you can, otherwise use something like Paralles, VMware Fusion, or Virtual Box that lets you set a 'drive' that grows in size so it doesn't use up your storage as fast.