I was told that you basically want half your hard drive free that is running the os. So if you have a 100g hard drive that your OS is on you should not use more then 50g. If you have a second drive you can max it out. Also if you get a second drive i would suggest that you put page file on that drive.

The half free is something i heard when i first started using computers, about 10 years ago, where 5g was a lot. So things have probably changed, but the sizes of hard drives these days i don't come close, i just store all my music, video, and stuff on a portable hd.