Core 2 Extreme parts are not supported on the 780 MT because the power supply and motherboard voltage regulators are too small. The PFC power supply is 305W and the Energy Star Unit is 255W.

Those processors work fine on Dell Precision T3400 which comes with 525W power supply and supports 2 video cards.

 The TDP for core 2 EXTREME series is 130W.  Max CPU on the 780 is 95W

https://ark.intel.com/products/35428/Intel-Core2-Quad-Processor-Q9650-12M-Cache-3_00-GHz-1333-MHz-FSB

The 9775 TDP is 150W

https://ark.intel.com/products/34692/Intel-Core2-Extreme-Processor-QX9775-12M-Cache-3_20-GHz-1600-MHz-FSB

 Intel® Core™ 2 Quad Q9650/3.00GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9550/2.83GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9400/2.66GHz, 6M, 1333FSB
Intel® Core™ 2 Quad Q8400/2.66GHz, 4M, 1333FSB
Intel® Core™ 2 Quad Q8300/2.50GHz, 4M, 1333FSB

http://i.dell.com/sites/doccontent/business/solutions/engineering-docs/en/Documents/optiplex-780-tech-guide.pdf

http://www.dell.com/downloads/ap/products/precn/precision_t3400_au.pdf

Excellent information.  Exactly what I needed to know.

Thank you!

"Currently the E8500 CPU at 3.33GHZ is installed."  <--- wrong... The E8500 works at 3,16 Ghz, not at 3.33. The E8600 CPU is working at 3.33 Ghz… Cheers