Buy an external hard drive and copy data files to it (especially big ones like movies) until at least 15% of your internal drive is free at all times. If you do not do this your computer will crash and possibly corrupt your hard drive. If that happens you will end up spending a lot more money trying to recover it than spending $100 on an external drive now.
By the way, you need to correct your computer information. You say you have a PowerMac but the PPC generation of computers cannot possibly run OSX 10.6.3 which you claim it has.