One more thing: there’s a difference between Nitris (older box, won’t work with software newer than v5.5) and Nitris DX (different box, different cards, does work with v3 and later).
Hi there, a G5 cannot be an Intel, and vice versa. As you write Xeon, you are probably meaning an Intel-based Mac Pro. Check the Avid Version Matrix to check what OS version and drivers you should be using: https://avid.custkb.com/avid/app/selfservice/search.jsp?DocId=267087
MC6 is very stable, considering it is a .0-release. Rule of thumb is to upgrade only when there are new features you need, and not rely on .0-releases for mission critical operation.