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How do I install more memory? I have a first generation MacPro. 2x2.66 Ghz Dual core Xeon 1GB 667 Mhz DDR2 667Mhz FB Dimm

Glad you asked before you did it. Here’s the PDF from Apple on how to properly install your new RAM: http://manuals.info.apple.com/en_US/MacP

Thank you Mayer !! A little more help would be appreciated: I have downloaded the PDF file and inspected my MacPro. The two Dimms are in the A riser (upper)in the first two slots. I bought two 1GB Dimms. Which slots do I put them? I wish to keep the two Dimms that came with the unit.