2006 Mac Pro (MacPro1,1) Flushed to 2,1 firmware Sapphire Radeon HD 7950 [Purchased January 2, 2014 for $468] Two Xeon X5355’s [Purchased February 25, 2014 for $45] --> Upgraded to Xeon X5365's [Purchased November 12, 2015 for $90] Macosxbootloader v3.1 OS X 10.11.6 El Capitan 64GB RAM (8 x 8GB) [Purchased November 18, 2015 for $199] - - - - - Caution: The type of DIMM that works in 2006 Mac Pro is PC2-5300F 2Rx4 FB-DIMM ECC. (1) The F after 5300 (or FB in FB-DIMM) means that the module is Fully Buffered. (2) DO NOT get PC2-5300P DIMM’s. These parity modules are incompatible with MacPro1,1; both physically and electrically. On eBay, use a search string like "64GB (8x8GB) PC2-5300F FB-DIMM”. I just did (November 21, 2015), and $176 is the lowest price at which you can get 64GB right at this moment. Here are the search results.