The previous system folders are just taking up space and can be safely deleted

The only way you can get more HD space is to trash some stuff. The trick is knowing just what stuff you can trash.
As you have obviously done an Archive and Install a couple of times, you may be able to regain some space by deleting the extra languages installed on the machine. By default, all the extra languages are installed but, unless you have developed an aptitude for Traditional Mandarin, you can get rid of it. There is a nice tool that will do it for you, monolingual, which can be found here:

If you are not using the MS Office Test Run (the trial version of Office) trash it too.

Do not get adventurous and start deleting files from any of the library folders, the system folder. The system folder is only about 1.4 GB and should be left alone.

You might want to consider purchasing an external, Firewire, hard drive to move some of the stuff off your machine.
iTunes and iPhoto take up a lot of space, as do video files.

Hope this helps