Go into the BIOS F10 setup. Look at the first page (System Information). This page lists the BIOS boot block date.
If the BIOS boot block date is 03/06/2013, the Z620 supports the E5-26xx V2 CPUs and the older versions (Ivy Bridge and Sandy Bridge).
If the BIOS boot block date is 12/20/2011, the Z620 supports only the E5-26xx "V1" CPUs (Sandy Bridge). The system will probably not boot with a V2 CPU, and even if it does, the system will be very unstable.
The Z620 boot block cannot be updated. Updating the BIOS to the latest version from older versions does not update the boot block. There are several threads in this forum about the boot block. HP does not support updating the boot block, for security reasons.