Re-found this cool commandline utility today, hfsdebug. Find out everything about your hard drive that you never wanted to know.
Once you’ve got that, get ShowVolumeFrag and find out the sad truth about your hard drive (in my case, most fragmented files were the most accessed, being the swap files :() Sadly I’ve decided to wait until after WWDC to rip my PowerBook open and upgrade the drive. Probably will need the extra space in order to fit Leopard on there.
We use hfsdebug very frequently, it’s a great utility when working with HFS+.