Per the MS website, XP requires 1.5 gigabytes (GB) of available hard disk space* minimum, while Vista requires 40 GB of hard drive capacity with 15 GB free space.
I have an IMAC C2D, and I've been running XP on a Fat32 formatted partition. Formatted mine to 25 Gigs. Plenty of space for my games and apps. I wonder what other people have formatted their XP partition size to???