I searched the forums and could not find any definite answers on this. I currently have a 4,1 early 2009 single processor MACPRO. I purchased the following items to upgrade to a dual processor MACPRO. Call me nuts but enjoy the challenge. 1. Dual Processor Tray 2. Dual SIX core processors 3. 1333 MHZ MACPRO Compatable Memory 4. Heat Sinks for both Processors. 5. New ATI Video Card for MAC. I already installed the video card and upgraded the firmware with the standard 5,1 netkas hack. Will the upgrade above work? I am waiting on heat sinks tomorrow.