Obviously I should have done some more research before buying the 2006 MacPro but I didn't and it was cheap and I can live with 10.7 if it must be so but I believe it's possible to run Mavericks on it with a bit of faffing. I have just replaced the G5 with a 2006 MacPro 1.1 (also 3.0GHz 8 core) and want to run Mavericks on it (the same as on the 3.1 and my 2011 MacBook Pro). I hav a MacPro in my studio (2008 3.1 3.0GHz 8 core) and until recently a PowerMac G5 in the office.
