For that matter, I would also love to hear about 16GB FB-DIMMS but 2 of those are worth more than any of these machines so I won't hold my breath waiting for news. I noticed a thread at Apple Support forums from someone using 8GB FB-DIMMS in a 3,1. He was able to get 64GB working in 3,1 but at the reduced speed of 667 Mhz. As we have a 1,1 and a 3,1 testing machine here I decided to try some 8GB FB-DIMMS to see what worked. And all I have found so far is what DOES NOT WORK. I got 2 separate 16GB kits. I have been unable to get either of them to work. It would be great to figure out what exact memory will work, if for no other reason than that future enthusiasts won't waste time & money trying things that don't work. One kit I got is labeled "Hynix 8GB 2Rx4 PC2-5300F-555-11 HYMP31GF7AFR4C-Y5 D5 1209" The other set is labeled "HYS72T1G842EFA-3S-C2 8GB 4RX4 PC2-5300F-555-11-Y0 D0 ZSL84303051" I have tried with existing sticks in other riser, with just these alone in a single riser, with just these together on same riser. No matter what, the red lights illuminate on the riser indicating problem with these sticks and they are 100% invisible in System Profiler. I have tried them in the 3,1 and in the 1,1. They completely, 100% DO NOT WORK. It is possible that one or the other set is bad, but I doubt both are. They were purchased on Ebay from separate vendors. They did not claim Mac Pro function, but the specs seem to match. It is quite possible that there is a certain exact model number that works. If so, it would be great to figure out what it is. And before the "Debbie Downer" crowd comes out to point out that this is too much money to spend on these elderly machines, I KNOW THAT AND DON"T CARE. I just want to get an answer to what RAM could be used for someone wanting an "Extreme Makeover". Might also be useful for someone who works with servers getting de-commissioned who might have access to some FB-DIMMS to try with. So, yes, I know that there isn't a single one of these machines that is worth as much as 8 of these 8GB sticks. Understood. But sometimes people just want to know.