You can free up a ton of hard drive space on your existing system by several methods.
Do not try any of these unless you have the means to make a complete backup of your system first, so you can restore any files that you delete that you decide you must restore later.
1) Run MonoLingual to remove Mac OS X language and help files for languages that you do not speak. This can free up as much as 1GB on some systems. Be careful.
2) Delete the contents of /Library/Printers/. This can free up another 2 to 3GB in some cases. When you next reboot and need to print something, Snow Leopard or Lion will prompt you to download the drivers for just the models of printers you actually use.
3) iDVD, iMovie, iPhoto, and GarageBand have huge support files, many GB in size, full of themes and sound libraries and other things you can temporarily do without. If you have your original DVD installer for iLife, you can re-install those later. Manually remove these support files, which are found in places including
Total savings, another 3 to 5GB.
4) Use the free OmniDiskSweeper program to examine your hard drive and find all the large directories of files to look for other things to delete. Be very careful and make sure you have a backup of anything you are eliminating.
5) Empty your Trash. Some users put many GB in the Trash and never empty it. That's disk space you should free up.
6) Be aware of other user accounts on your hard drive other than your own. Under those accounts there may be huge cache files that need clearing, Trash that needs emptying, or large documents or directories of files you can archive and then delete.