Options:
1. For a virtual machine, you probably want an old copy of Virtual PC, v6 or v7. This will cost money.
2. Virtual PC will cost you more than buying a second hard drive. So buy a 2nd hard drive, it'll serve you well after your experiment with Linux. Not quite sure why you bought a G5 if you weren't planning to use the expansion.
3. Backup everything you want to keep, reformat into several partitions and re-install. Put Linux on the first partition. This will be free.
4. Sell the PMG5 and buy an old iMac G5 and Virtual PC, or an old iMac G5 and an old Intel PC. Better than free, you'll probably make some money on this.
5. Look for a free old hard drive from a work colleague upgrading their Mac / PC. Can't remember the number of disks I've parted with to friends, from 4GB to 10GB, a batch of 60 and 80GBs and even more 120GBs. Make sure you get the right kind (SATA) for your PMG5. I think these start around about 160GB. Cost between free and a cup of coffee / tea.
How big is the disk you use for backups ? Or do you backup to DVD / CD ?