well, if you are planning on taking an Image of the SYSTEM partition then use about 8 GB. Then you can get XP on it and have room for allof the upgrades. Then you need to use a different partions to actually install your apps to it, as after a while you will run out of space and need more room if you put all your apps on the main dir. Programs that you use alot need to go there, but games and really big stuff need to go somewhere else.
If you are not planning on using an imaging tool, then Partitions are really useless. The only reason to use a partition now is to limit the size of the total space. There is not a reason to use it if you are not planning on capturing the config to restore later.
My opinion.