This forum post should help.
The DT power supply does not support Quads.
You are limited to
Intel® Core™ 2 Duo E8600/3.33GHz, 6M, 1333FSB
Only the Tower Does.
The Optiplex 760 MT uses two motherboards -
T674K Non-Quad core CPU motherboard
U562N Quad core CPU motherboard
Intel® Core™ 2 Quad Q9650/3.00GHz, 12M, 1333FSB
You you cannot use a quad core CPU on it.
38G73 E7400, 2.8GHz, 3MB, WFD, R0, (With VT)
F31F5 E7500, 2.93GHz, 3MB, WFD, R0, (With VT)
06YMG Dual Core E7600, 3.06GHz, 3MB, R0, (With VT)
H399J Dual Core E8400, 3.0GHz, 6MB, 65W, E0, 1333FSB
D923J Dual Core E8500, 3.16GHz, 6MB, 65W, E0, 1333FSB
N654J Wolfdale Processor E8600, 3.33GHz