I know next to nothing about RAM.
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.
Mix and match brand or size? Previous owner had mixed 4gb and 2gb sticks but told me its not good to do, and so maybe it hasn't been optimal.
Both. Just max it out once or get half the sticks of some RAM that will be around for a while.
Yup. Not good to mix and match anything for registered stuff. It's best to just use all of the exact same memory.
Or just stick to the qualified vendor list (QVL) for parts that the manufacturer has certified to work with your system and you should be ok.
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.