Auqa,
CPU: The Xeon X5677 should give good results over a wide range of uses. We have three systems in the office using Core2 CPU's with single-thread rating under 1300 and with a good GPU, these are still very good visualization and research systems. I acquired a new respect for LGA1366 when I learrned that the local particle research facility uses 2010, single CPU LGA1366 systems to model particle accelerator modules in Siemens NX - with Quadro K6000's.
In terms of the Xeon / iCore equivalency, the Xeon X5677 average Passmark rating of 7005 is similar to the i5-4570 with 7010, the i5-6500 at 7012, and the Xeon E3-1225 v3 at 7022, and i7-2600S with 7045. Those do have quite a bit higher single-thread ratings, but the calculation density is about the same.
RAM: Wiith regards to RAM, if you're adding RAM, it should use the maimum speed for the CPU- 1333. The selection depends on your uses and the z400 would have ECC error-correcting, but can use non-ECC. If the work is analytical or includes video editing or rendering, use ECC.
If you use several programmes at once and /or have large file sizes, I'd recommend 6X 4GB. In our work, there are often several things going: 2D CAD, 3D CAD, photoediting, graphic design, Win Explorer, backup, browser, and file sizes can be large, we can have 4X 160MB CAD files and 20X 300MB photos open. The two types can not be mixed. ECC does a parity check to enusre high levels of precision, and is very slightly slower. The maximum is 24GB and I'd say 12GB is minimum. It's very inexpensive used and my suggestion is to have either 3X 4GB or 6X 4GB.
Memory in workstations can be fussy. Embarassingly, I had to buy RAM for a 2007 Xeon X3230 system three times before it worked properly. In our LGA1366 systems, the X5677 one arrived with 3X 2GB and was upgraded to 6 X 2GB using the same, exact model RAM. The Dual X5680 systems has 6X 4GB for CPU 1 and 3X 8GB for CPU 2 on the riser board- balanced quantity between the two CPU's. Again, the 8GB modules added for the 2nd CPU were as close as possible to the original 4GB. There is a specific layout for RAM in the manual, If you mix module sizes, place the larger three in the first slots. It is best to use the same module throughout, but as long as the channels are filled with three of the same one, the system can have different sizes. All the systems here except one are using Samsung and when a system has original memory, it's best to try and find that same brand and series. When one of the z420's added RAM, it was Samsung with an HP part number. - Given the low cost, I'd say have 6X 4GB- the maximum, and if it's done properly, you'll never need to think about RAM again.
GPU: Coming from the world of Quadros and the occasional Firepro, the GTX 750 Ti is really a great bargain. The GTX 750 Ti for US $120 has a Passmark 3D GPU average of 3862 while the Quadro K2200 at 3442 cost $450 new
You might like to have the 30-day free trial of Passmark Performance test, run the z400 as is, and then compare the results with the upgraded version. It's very useful to identify weak components or settings that can improve it, e.g., the often overlooked "High Performance" setting in Control Panel /Power Options and avoiding interface effects like transparency and animation, a lot of scheduled tasks, and hibertation / sleep modes. I only ever use Basic Theme as I found Aero and themes reduced overall graphics performance by 15- 20%.
Cheers,
BambiBoomZ