Registered memory (aka server memory) is more expensive than desktop memory. If you want to add DIMMs to the existing ones, make sure to match the timing as well as the basic type (e.g., DDR3, triple channel for the z600, with six DIMM slots on the motherboard). Timing will be listed as something like 9-9-9-24. If you install DIMMs with mismatched timing, it will cause data flow traffic jams, slowing everything down.
The safest bet is to remove the existing small-denomination DIMMs (1GB sticks, e.g.) and buy a replacement set of
used server DIMMs via eBay. So for the z600, six 8GB DDR3 DIMMs will max it out - the z600's motherboard main chipset will only read up to 48GB. Six 4GB DIMMs would give you 24GB total memory. It is OK to have DIMMs with different capacities such as a mix of 1GB and 2GB sticks, but the timing needs to match.
I see
24GB sets for the z600 on eBay for $40.50 which have the metal heat dissipators.
As for processors, the z600 will take two Xeon Westmere-series hex-core processors of 95W heat/power rating; it will *not* boot if you install two of the 130W Xeon hex-cores (I've tried it). So, the X5670 is an excellent choice. The X5675 is the fastest which is compatible with the z600.
Core 2 Quad and core i7 CPUs cannot run in parallel; you can physically install two on a server motherboard, but it will only read one. Use Xeons if you want to run dual CPUs.
For anyone reading who has a bigger budget for new gear, dual-CPU motherboards for AMD will need
Epyc series processors (not Ryzen/Threadripper). Only the Epyc CPUs can run in parallel. If you want Intel CPUs, get Xeons. I have heard rumors -unverified- that Windows has a thread limit of 64. If (and I stress *if*) so, then I would get a pair of the fastest clockspeed HyperThreaded 16-core (or fewer) Xeon processors.