Should I underclock/undervolt?

Hello,

I've had my 4670k overclocked to 4.2ghz since I built my rig around a few months back, temps are actually pretty good and I'm using a Dark Rock 3 to cool, and I was getting curious about actually underclocking things below stock speeds and get things running extra cool, if possible. How should I go about this? Will I lose a lot of performance (realistically)?

You will lose performance if you are doing things in terms of rendering and steaming but probably not too much in gaming. Perhaps under clock it to 3.0GHz and just increase the speeds until gaming is very comfortable for you.

kind of curious why would you want to do that if everything is running fine and dandy?

Try and find a balance of low voltage vs clock speed.  The voltage is what lowers the temps mostly.  So see what the lowest voltage you can get at like 4ghz or something like that.  That is what I did with my 3770k since I have it in an itx case and need to keep heat down.  3.9ghz @1.1v