Here's the deal.... you are not expected to know this, but there are two types of Z600s (and two types of Z620s). The originals, and the "revision 1" builds. The easiest way to tell what box you have it to look at the BIOS boot block date and compare that to the info you'll find in this forum to tell whether you have an original or a Revision 1.
If you have an original, which I believe you do (I can never remember the exact boot block dates for the two Z600 types) then you are stuck, and can only run the processors that HP certified for the original. This is not a big secret, and it is all searchable in this forum, but you will need to take the time to educate yourself.
A revision 1 Z600 can run all the processors; an original Z600 can only run the older set of processors.
The neat thing about that generation of workstations is that it is the last where it was very easy to add a second processor (versus all the gear you need to add a second to the Z620). HP always said it would be easy to add a second identical to the first. They never intended for us to dump the first and add in two of a faster processor. Read up also on "microcode". Use that search box at the top left.... it only searches this forum. Lots of great posts were lost when HP split itself.
Good luck on your self-education.... if you lust after a revision 1 Z600 box (I wish they'd called it Generation II) then you need to consider getting a Revision 1/Generation II motherboard and swapping that in. That is all possible if you are handy... it is pretty easy, actually.
