Just finished a full Z600 upgrade build to the two fastest processors, plus more. Here is current information:
1. Critical to know if you have an original motherboard or a "Revision 1" motherboard which allowed using the better faster processors. Read up in this forum on the Z600 boot block date, and if you have the later one then you are in good shape. Data from the 2009 brochure provided refers to the old original motherboard and less capable processors.... you need to look in more recent data sources if you have a revision 1 motherboard for accurate information.
2. Ergo, search for the HP QuickSpecs such as from 2013: http://www.usedcomp.de/pdf/HP-Z600-Workstation-QuickSpecs.pdf There you will find the more recent listing of the HP approved processors for this workstation (assuming you have Revision 1). I think the latest I found was version 51 of the QuickSpecs. I ended up getting 2 of the X5675 hexacore Xeons, which run at 1333 FSB and this results in your memory running at 1333MHz also assuming it is rated for up to 1333. Otherwise, unless you use a 1333 processor your memory will only run at 1066. The 12 cores total cost me 190.00 USD off eBay. There is only one sSpec code for that processor so if you search for SLBYL there you'll see the price ranges. Better than buying new at 1440.00 each (6 cores). Yes, $2880.00 USD total, if you wish, versus 190.00.
3. I prefer HP memory because it is binned to be better than stock. It is best per HP to run identical sticks, and I chose 6 x 4GB for this project. I'd be fine with 6 x 2GB sticks usually. You'd probably want the 24GB total, and supposedly you can run 6 x 8GB also. I happened to end up with the HP Samsung version of these sticks, but there are Hynix, etc. Cost was about 100.00 total, again from eBay, with careful shopping.
4. I added in a HP Texas Instruments USB3 card, as posted in this forum about a week ago. In that post is added info on my favorite eSATA backplane adapter.
5. I'm running a 300GB Intel 320 series SSD... this is a SATA generation II workstation, and HP has recommended sticking with SATA Gen II drives for such workstations unless the drive is a SATA gen III from HP, with their special firmware applied. Another eBay buy.
6. There is a new 3.60 BIOS from April 2016.... be sure to load that before anything else. Best to update BIOS from within BIOS, which is posted about in this forum also.
End product is a Z600 that is running with same scores as the fastest Z620 (revision 1 Z620) that I have access to. I have limited out this Z600, and the Z620 has more headroom, but am very happy with the end product.