Well for games they say on the box how much HD space they need, so add all of that up and add about 2 GB for Windows install and probably another 3 Gb. For that amount of games you are probably looking at least 40 GBs
XP uses about 3GB for installed files, Vista about 7GB (needs 15GB to run installer). Windows doesn't run as well with a nearly full disk, so leave some extra space.
A good starting place is 20GB for XP and 32GB for Vista.