Ok, to the heat issues and the importance of cleaning the dust from inside your PC:

It's been too long since I cleaned mine last, picked up some canned air today and went to to town. PC off, of course, also was careful, as suggested, to prevent any fans being sprayed from rotating with my finger or a pen (rotating the fan with no current coming to it can produce a current back into the system).

I wish I had tested the temp right after the dusting and before touching anything else, so I could have seen just the difference from dusting. However, I did also adjust my fan speeds on the case, increasing the speed on the fan that pulls air out of the case directly behind the heat sink and slightly lowering the fan that exhausts out the top blow hole. I also suspected that over time the thermal compound between the Heatsink and the CPU may have developed air pockets, so I gently pressed down on the heatsink and wiggled it a tad. (I probably should have removed it completely, cleaned off the old compound, applied some new and re-installed the heatsink, but it seems to be working great now so I'll watch my temps for any increases in the coming days/weeks).

Rendered the same scene at high render settings on both cores, ambient temp in the room is about the same.

CPU temp went from 59C (under full load) before cleaning and adjusting to 44C under full load after!

The change was so significant I'm sure it has to have been from more than just removing the dust, but it definitely illustrates the value of cleaning the inside of the case on a regular basis. Temp at idle decreased a lot as well, from 41C to 31C.