Once your HD reaches the 20% free mark, you are gonna see significant slow down.
Word to the wise...your boot drive (the HD your PC boot's off of), should hever be more than 50% full. As a matter of fact, I always tell people to just put in a boot HD for strictly programs...and buy a SEPERATE non-bootable HD for all your media (music, movies, etc). This way, nothing gets cluttered.