On eBay, here are the DIMM's I purchased 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, and $176 is the lowest price at which you can get 64GB right at this moment.
Here are the search results.
Awesome thanks for sharing this info. I'm tempted to upgrade before the end of the year