REAL ballpark, 4GB for XP, plus all the games, plus 10% rounded up for swap. So, if you have games that add up to 10GB, then 4 + 10 = 14, plus 1.4 (rounded up to 2) equals 16GB. Honestly, that still may be shy.
I'm melding my Dell from work and my iMac from home into a single MacBook in the next 30-90 days. The Dell's currently got 19.2GB used for XP SP2, MS Office, and work-related apps & utilities used up, so I'll give the MacBook probably 30GB, leaving 90GB for the HFS partition (using ~40GB now). This is, of course, predicated on the assumption I stick with the base 120GB HDD.
The main problem is that you CANNOT resize partions on the fly with a mixed HFS/FAT32/NTFS environment. I think you'd rather have the Windows partition a bit on the large size than the other way around.
Either move to a much bigger drive, or plan on uninstalling games periodically. Some of them can get huge (MS FSX, Far Cry, etc).
Bon chance!