The performance difference between Westmere (X5000 series in the Z600/Z800) and Sandy Bridge (E5 used in the Z620) looks negligible.
The high end X5670 or X5690 should outperform all but the top first gen Xeon E5.
It uses more power at idle, but the upfront cost is almost halved at the moment...
I'd say you should get a unit with the highest end processors you can find (it will end up way cheaper than upgrading them) and preferably with the 1100 Watt PSU, the rest can be upgraded later and you should have plenty of power for anything (dual GTX 9xx for gaming and stuff :-))
That's not correct. Sandy Bridge was a huge performance jump over Westmere, with ~50% performance for most applications on equivalent models between these two generations. A first gen E5 2640 should handily outperform the X5690, and a current gen (v3) E5 2620 would even outperform that.
http://cpubenchmark.net/high_end_cpus.html
Obviously, the upfront cost of the older hardware is still better value when ignoring power consumption, but the v3 will also support much more and faster RAM, PCI-E 3.0, USB 3.0, and other newer technologies which will matter more as time goes on. You'll also have the option of a CPU only upgrade to higher end E5 v3's or even E5 v4's at a later point in time when those CPU models are more affordable.
I totally missed the fact that the HP Z620/820 supports v2 and v3 E5s. That definitely makes it a better choice for future upgrades.
The X5690 is comparable to the first generation E5-2640 (slightly slower), and the X5670 is comparable to the E5-2620.
Even the first gen E5 has the AVX instruction set, 4 memory channels (vs 3 for the Westmere-EP) and a faster QPI, so it might have better performance in certain applications.
Sorry, I didn't meant to imply the Z620/820 supports v3. Just pointing out that the performance jump in Sandy Bridge was significant.
v1 and v2 are socket compatible, but v3 is not socket compatible with the earlier versions, but will likely be compatible with v4 (Intel socket changes usually happen only every other generation). I'd suggest going all new with even a low end E5 2620v3 if you can, which you could for everything other than the GPUs for under 1700 (based on a Supermicro workstation build).
The high end X5670 or X5690 should outperform all but the top first gen Xeon E5.
It uses more power at idle, but the upfront cost is almost halved at the moment...
I'd say you should get a unit with the highest end processors you can find (it will end up way cheaper than upgrading them) and preferably with the 1100 Watt PSU, the rest can be upgraded later and you should have plenty of power for anything (dual GTX 9xx for gaming and stuff :-))