I'd recommend a nice 10 GB. Gives XP, the applications, and your files plenty of room to breathe.
XP cant read HFS+ volumes, without some 3rd party software.
Also, with XP, the more RAM you have, the bigger your swap file is.
Some steps to take when you get it installed, disable System Restore. It is USELESS. When you take it back to a point, it loads up old files, and will erase parts of applications you've installed since then, leaving a big mess, and it will sometimes delete data that you have made since the restore.
Also, another feature is hibernation. Also useless unless you are running it on a MacBook Pro, but even still, its kinda useless. Hibernation dumps the contents of the RAM to hard disk, but still takes about as long as normally booting XP, since it has to read the entire RAM dump in, even if the RAM wasnt being used. T_T
Its quite a dumbass system.