You will have to update bios to A22.
Cant do this on all models. USFF DOES NOT SUPPORT QUADS.
BIOS - BIOS
| Description | Importance | Version | Released | Supported OS | Download |
| Dell OptiPlex 755 System BIOS A22 Dell OptiPlex 755 System BIOS A22 | Recommended | A22 | 1/4/2013 | BIOS |  |
validated -
N652J 3.00GHz Yorkfield Quad Core Q9650, 12MB
F886J 2.83GHz Yorkfield Quad Core Q9550 12MB, 1333FSB, E0
N650J 2.66GHz Yorkfield Quad Core Q9400, 6MB, 1333FSB, R0
MP627 2.66GHz Kentsfield Quad Core Q6700, 8MB, 1066FSB, G0