HI,
I am also on a G5 with a PT accel HD3 (PCI) system - at the moment running VePro V4. (the G5 runs just ProTools, and I have Win7 64 bit PC with VePro hosting all my virtual instruments.) I'm in the process of upgrading to 5, but have updated ViPro 2 first and am integrating that into my VePro4 templates first, before actually doing the V5 upgrade. If you haven't used VEPro before, it has breathed new life into my aging G5 ProTools rig - big time! Amazing piece of software! I don't anticipate any problems with V5, but am just going slowly. So, I guess I'm in the same boat as you - I <will> be upgrading, but just waiting to see what issues (if any) need sorting out first. But V4 has been just rock solid.
Not exactly an answer to your question, but wanted to share my 2 cents worth, since we both are running the same type ProTools rig.
Best,
Greg