Upgrade Dell Optiplex 745

The 965 chipset found on your Dell is NOT one of the listed compatible chipsets with an E8400:

http://ark.intel.com/products/33910?wapkw=e8400

http://www.google.com/url?sa=t&rct=j&q=dell%20optiplex%... (PDF file)

Yes no? What is the fastest processor I can upgrade to please help and thanks

Intel Core 2 Extreme X7900 - 2.8 GHz Dual-Core Processor SLAF4 what about this one

The X7900 is a mobile processor, not a desktop processor... Your mobo uses a 'Q965' chipset, so you have to find a compatible processor for that chipset. Here is a list of compatible products for the Q965 chipset:

http://ark.intel.com/products/chipsets/22756

Of those listed, the Core2Duo E6600 apprears to be the best CPU you can put on it:

http://ark.intel.com/products/27250/Intel-Core2-Duo-Pro...(4M-Cache-2_40-GHz-1066-MHz-FSB)