N640GT :
0G214D Q45/43 Intel Southbride 82801JD
that is a core 2 duo, or core 2 quad motherboard
so not really worth upgrading, these will work though
38G73 E7400, 2.8GHz, 3MB, WFD, R0, (With VT)
F31F5 E7500, 2.93GHz, 3MB, WFD, R0, (With VT)
06YMG Wolfdale Dual Core E7600, 3.06GHz, 3MB, 65W, R0, (With VT)
H399J Wolfdale Dual Core E8400, 3.0GHz, 6MB, 65W, E0, 1333FSB
D923J Wolfdale Dual Core E8500, 3.16GHz, 6MB, 65W, E0, 1333FSB
N654J Wolfdale Processor E8600, 3.33GHz
E8000 series Intel® CoreTM2 Duo 6M, 1333 FSB
E7000 series Intel® CoreTM2 Duo 3M, 1066 FSB
E2000 series Intel® Pentium® Dual Core 1M, 800 FSB
E1000 series Intel® Celeron® Dual Core 512K, 800 FSB
400 series Intel® Celeron® 512K, 800 FSB
for the core 2 quads to work you need U562N mobo:
Q9550, Q9650, Q9400, Q8300, Q8200