Actually, the Dell Optiplex 745 can handle a lot more than this. I used to have one that happily ran a Core 2 Quad Q6600 CPU without a hitch, and the closest I got to custom modification was replacing the thermal paste with Arctic Silver. My only limitation was lack of space for a half decent GPU that could match it's power which wouldn't be an issue if the Optiplex 745 weren't the right hand drive vehicle in a country full of left hand drive ones.
Also, the most RAM it will support is up to 4x2 GB DDR2. I think it can support higher than 800 mhz but I don't really remember...it's been a while.