The two cpu have same TDP of 84w.[URL] If you have the 65w heatsink now there is option to upgrade to 95w heatsink or heatsink of copper core optiplex XE2 which can support i7-4790k.   the 65W heatsink is thinner  [IMAGE] the 95W heatsink is thicker  [IMAGE]  
Advice, please. My PC came new with a Intel Core i5-4460 @ 3.2 GHz 4th Generation CPU.  I am thinking to upgrade it to Intel Core i7-4790 @ 3.6 GHz 4th Generation. Would I need to change the Dell heat sink and 64CFM CPU cooling fan already installed? Many thanks.      
Vostro 3900G MT CPU Upgrade