Seems to me you are contradicting yourself somewhat.
I have never seen it stated exactly how much free space needs to be maintained to avoid any slowdowns, if you have please share your source.
http://www.speed-up-your-slow-computer.com/free-up-disk-space.html
For the OP to save $30 on a 320gb vs 500gb seems kind of of foolish. Just my 2¢
If you are under 1 gig left, then you will start to see some slow downs. But anything over that you will not. The only time slow issues might happen is when the HD is all fragmented.
But just putting stuff on the HD will not slow it down till you almost takes up all it's space. Nothing contradicting about my statement at all.