I have an HP z800 and had been following some 10600R ECC REG RAM on ebay and the price just suddenly skyrocketed.
Some Samsung 2RX4 PC3-10600R DDR3 1333mhz Only ECC Server Reg Memory is on sale on NewEgg but I'm not sure if it will work on the z800. Google isn't helping on this (guess not that many people mess around with z800s).
This may help http://h20331.www2.hp.com/Hpsub/downloads/Mini_WP_Z800_memory.pdf The Z800 supports Registered ECC DDR3 dimms, so those Samsung dimms that you saw on Newegg should work without issue. However, if you plan to mix and match Registered dimms, that usually causes issues, and some workstations/servers will not boot with mix-and-matched memory.
Did a disservice to this thread and didn't post links:
The ebay ram specifically for the z800 that went from $170 to $232 overnight...
The NewEgg RAM
Not that much detail on the ebay RAM but looking at @TheAlmightyBaconLord 's PDF link a 64 gig setup isn't as optimal than a 48 gig, in fact a 64 gig setup is not even in the chart...
Also with some google'ing, am I right reading into, "HP Z800 supports Registered 4GB and 8GB dual rank PC3-10600R and PC3-10600P 1333MHz DIMMs" as good to go for 2RX4?
The price on the NewEgg RAM is great in comparison to how the ebay RAM jumped. I'm aiming for 48gigs, at 24 gigs I was running out of RAM.
just a note, current 24 gigs is a mix of a few brands, 16 gigs made up of mixed brands of 2GB 1Rx8 and two sticks of 4GB 1Rx8- wondering if this put a speed hit on the RAM, hope things are optimized by getting nothing but the NewEgg/Samsung RAM in there.