virtualisation is one possible method of doing what you want but, since the machine you're talking about is a G5 PPC Mac (i.e. quite old and slow by today's standards), AND it sounds like you're using it as your main workstation as well ("do other stuff") AND it's intended to be a production server, IMO a far better option is to pick up a second-hand P4 or better PC and install linux on that.
Getting linux installed and working on a real PC will be a lot less hassle than getting it running in a virtualisation environment on hardware that is both ancient AND a completely different CPU/architecture/platform (virtualisation sw, if you can find any for a PPC Mac will almost certainly emulate an i386 PC rather than run native ppc code)
Given that you can buy brand-new 64-bit AMD or Intel headless whitebox clones with 4GB or 8GB of RAM and 500GB or more of hard disk space (and Gb NIC, IDE, SATA, etc) starting from about $500 AUD (about $400 USD) these days, you can probably get a 2nd-hand P4 with 1GB RAM for $200 or so. or recycle someone's obsolete-but-still-overpowered desktop machine.
depending on how much load your production server is expected to get, you may even get away with an old Pentium-II or something - you can pick them up for free, although they're less likely to have a built-in network card (will cost $20 or so)
Also, AFAIK, Virtual Box only runs on Intel Macs, not PPC. I think Parallels does too. Microsoft's Virtual PC for the Mac ran on PPC. if it's still available to purchase anywhere, it will probably cost a significant percentage of what buying a second-hand or even new whitebox PC clone would cost. It also emultes PC hardware so you'd have to run an i386 linux rather than a native PPC linux.
don't bother. get a cheap pc to run linux on.