Processor Compatability for HP 4000 Pro

[Captain Beeble]

I have an HP 4000 Pro SFF that I have suped up ala the $69 gaming PC with everything except the quad core (and, ehem, a GTX745....)
I purchased a Q9550 per one set of HP specs and it threw out an 1802 error- only to learn they updated their specs for this machine and didn't include that chip.. SOOOO I purchased the fastest quad core identified in the updated list, a q8400 and now I am getting the same thing. 

This is my first foray into any sort of build, so apologies for any ignorance, but could there be something I am doing wrong, or would a dud-chip throw a 'processor not supported' error? Has anyone else dealt with this issue?

 

Thanks in advance.