Hi PeterSchwatz75,
As with DGroves and BambiBoomZ, I don't have a Z640, but regarding your questions I hope the following may help.
1) I know personally of 2 different occassions where either a friend or myself have tried to install a CPU not listed in the respective quickspecs from the same Xeon family in both the HP Z600 and HP Z620 workstations. On both occassions, the computers failed to boot. So I would have to say you definitely cannot install a CPU that isn't listed in your workstation quickspecs. However, see 2) below;
2) During the lifespan of HP workstations, component compatibility improves, i.e. through BIOS updates. Adding support for M.2 drives and UEFI boot for the HP Zx20 platform being a prime example, which occured when a later BIOS was released. To answer your question, you should only have to upgrade your BIOS if the CPU listed in the quickspecs was released by Intel after the date of your BIOS. If you have a very early BIOS release then a BIOS upgrade may possibly be required. Basically, I would stick to the CPU's listed in the quickspecs and try the CPU prior to upgrading the BIOS.
3) I notice your current CPU is only 85W but some of the other v3 CPU's listed in the quickspecs are 145W, which is quite a jump in power demand, and more importantly - more heat. Being unfamiliar with the Z640 platform and available heatsinks, I would strongly recommend you do some further investigation to determine if your current CPU heatsink is suitable for the higher power CPU's. e.g. there are various heatsink models available for the Z420/Z820 workstations - you need to use the liquid version for the E5-2687W CPU's due to their power demand and high heat output.