afaik Dell has not updated 790 bios to support ivy bridge Intel 3rd gen cpu. i7-2600 is the top sandy bridge cpu for 790.  i7-2600k is not better because Dell bios/chipset does not let you OC -k cpu. a search for i7-3770 in 790 user benchmark yields zero hit confirming this. the advantage of 3770 over 2600 is very modest.  Compared to its predecessor, Sandy Bridge, ivy bridge has 3% to 6% increase in CPU performance when compared clock for clock UserBenchmark: Intel Core i7-2600 vs i7-3770 in addition desktop ivy bridge cpu has the disadvantage of being more easily overheated. "Ivy Bridge's temperatures are reportedly 10 °C higher compared to Sandy Bridge when a CPU is overclocked, even at default voltage setting... this is because Intel used a poor quality thermal interface material (thermal paste) between the chip and the heat spreader, instead of the fluxless solder of previous generations"
Do you think my Optiplex 790 with a I3-2120, can be upgraded to an I7-3770 3.40Ghz, or do I have to go with the I7-2600 3.40Ghz, that is listed in the Service Manual?  The I7-3770 should be a little faster than the I7-2600.
Optiplex 790 CPU upgrade to I7-3770?