Don't be tempted. Massively regretting my decision TBH. I Have a dual 2.26 Octo and purchased 2 x Intel XEON X5670 hexa core to turn it into a 12 core monster. Haven't fried a board, haven't broken a processor, but haven't made it work either. Must have spent 30 hours trying over the last 3 days and am now just weary. It just seems to be pot luck as to what 'works' - no pattern to it at all. Have placed and replaced processors about 20 or 30 times now, but CPU B for me won't work at all. Tried swapping the chips over from A - B...same result. Put my original chips back...CPU B works, CPU A now won't work - WTF! Honestly it's just not worth the hassle. I've started over and have done incremental tightening of the CPU's in what would be 5 minute segments on a clock - still no nearer to success. Single CPU's upgrades are fine but Dual CPU's. I think the difference between those who have been successful and those who haven't is nothing more than serendipity! Had my first CPU installed up and running inside 30 minutes, the 2nd CPU well still trying after 3 days (sigh). So heed my warning. On a Dual CPU 4.1 Mac Pro..DON'T BOTHER! Keep your sanity.