You want to find a very late Z600 QuickSpecs and look at the fastest processor supported by HP for that workstation. My recall is that it was the X5690, one or two of them, but only if they had the 130W max TDP rare and expensive HP heatsink/fan atop them. Many if not most of the Z400 had a single 130W rated heatsink/fan in them, but that is bigger and will not reliably fit in the smaller Z600 case.... I tried without modifying that Z400 one.
The X5690 is a dual QPI link processor but its sibling the W3690 has only one QPI link. If you wanted to try one W3690 you might try an experiment. With a Vice Grips couldyou pinch flat the upwards progecting heat tubes that rise above the top surface of the Z400 130W Performace heatsink and fold them down flat to give you just enough room? I never tried that... stuck with the fastest 70W max TDP Mainstream heatsink/fan. The official HP Performance heatsink/fan for the Z600 are very hard to find.
We're still using a few Z600 v2 with 2xX5675 70W processors and with a nice fast SSD for both boot/programs and a second as a documents drive that is still a very servicable workstation. Building new today I like the Z420 Z620 v2 workstations better, however.
