I've seen this problem when the stepping on both cpu's does not match.
And your first screenshot seems to indicate the cpu's are not the same stepping.
Try pulling one of the cpu's and see if that allows you to boot 64 bit guests.
I just wanted to share the greatest experience i've ever had with a hardware company.
About 6 months ago i purchased a Das Keyboard on Ebay that was advertised as NIB
It arrived and worked OK but the keys where kinda slow and over two weeks got to the point where they weren't even...