my projects get burned to DVD as soon as I'm done with them, thus they take up ZERO space on my drive. I only create PT sessions when someone is interested in a beat, so they only exist on my drive for about an hour, just long enough for me to open FL, make detailed track notes, export wav files, open PTMP, import files, recreate the mix, tweak/audition, then burn three copies (master, backup, artist copy).
FL zip sessions take up minimal space, so they stay on the drive as work-in-progress files.
I have a pair of 320GB drives in my machine, the primary drive is about half capacity, the second drive is almost full, but that's mainly because I just reinstalled XP and dumped my "My Documents" folder on it and haven't deleted it.
True, though, I'm adding another drive to my machine this weekend, and it's a larger drive (~500GB), but that's mainly so I can use the second 320GB for a Linux drive.