G5's were never officially supported with mix systems, however some users on here have reported success in getting a few G5 models to work with mix cards.
In any configuration, the latest software that will work with Mix core cards is OS10.3.9 and Pro Tools TDM 6.4.1. Any OS or pro tools version after that will not work.
None of the G5's can natively boot in OS9, so you will lose your ability to use your floppy authorized plugins.... You need to find ilok versions of them if they are even available.
I think milesofwire got a mix system working with a dual 2.7 G5 a few years ago... I'm not sure if he used an expansion chassis or if the cards worked directly in the G5. Search the legacy section of the DUC, there's a thread on it somewhere in there.
Shameless plug: If you do decide to go this route and it requires a chassis to work, I currently have a G4/G5 compatible 13slot expansion chassis for sale here:
http://duc.avid.com/showthread.php?t=325796