not having a z640 i can only offthis observation
the z640 box's are very compact considering what they contain, as such heat and power draw can be a issue it's possible that cpu's which draw more wattage than what HP has specced will not run, and may give a bios error on startup to that effect
hp may also not want the box to compete with the z840 in terms of processor speeds and may block the faster cpu's because of this
to be safe when upgrading stay with the HP approved cpu's or at least make sure a unlisted cpu is the same (or less) wattage as what hp lists on their fastest cpu
depending on the software used,..... more cores rather than more "IPC" may give better results and sometimes stepping down one level in clock speed will save money and power draw, but only be 1-3% slower overall