Dell Optiplex 790 (sff) CPU Upgrade

Hey guys, what is the best cpu upgrade path which fits in no problem with the cooler that and doesn't overload the 240w psu.
Also I don't plan on buying a discrete gpu, just the cpu and its integrated graphics.
I'm looking at an i7-2600 right now since that's what everyone is going with and can I ask is it the top of the line cpu upgrade for the 790?

Im using a i3-2120 right now.
And can I also upgrade the ram? out of 4 slots 3 ( 2+2+4) and I'm planning on upgrading it to 16gb. Is dual channel recommended or is it okay to just fill all 4 slots (4x4) ?

Appreciate your recommendations and feedbacks!