I definitely need to clean it out...
Agreed. And as soon as possible. First I would make a backup/clone on an external firewire HDD. Then you can begin the work of slimming down the volume on the internal HDD. Run Repair Disk in Disk Utilities to be sure that your directory is in good shape. Repair it if necessary.
There are some things you can remove from the current installation that will give you a bit of breathing room. Dr. Smoke's FAQ
Freeing space on your Mac OS X startup disk will help you through that process.
If your directory is sound you don't need to reinstall. You can archive stuff you don't need to keep on the internal HDD on CDs, DVDs, or an external HDD. Eventually, however, you should consider a newer larger capacity HDD. The one you have may fail without notice, so be sure to have a backup that you keep current.
Good luck.
😉 cornelius