Brian,
Thanks much for that..... it really helps in making the decision.
One little thing..... regarding the memory change with the Version 2 of the Z400/600/800 that came out in 2010. It is my understanding that for Version 1 Z400, Z600 and Z800 memory was only unbuffered ECC and for version 2 the hardware changes allowed unbuffered or buffered (registered) ECC memory. Officially, I believe, ECC is what is to be used regardless. This memory stuff can get confusing.
There is a "system roll" document that came out for each of those three workstations roughly at the same time as they went from the B3 to the C2 stepping of the chipset. The link for the Z600 one is HERE.
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=c01982145
The Intel Ark page for the two steppings of that I/O Hub chipset can be found HERE with links to the specs to the left.
http://ark.intel.com/products/36783/Intel-5520-IO-Hub#@ordering