The PM G5s used serial ATA hard drives.These are still usable in some of the newer systems. So...
If your new system is a Mac Pro, then it will fit hansomely into one of the three empty slots.
For all other new Macs, you could still use it in an external enclosure from various manufacturers.
However, the OS (10.5.8) on your old drive will not run on any of the new Macs.
Availing yourself of either option i mentioned would allow you to use the old drive and data for migrating to the new Mac. While the folks at the Apple store would not be able to install this drive in your new baby, they would be able to help you with the data transfer should you require it.