Ok, jesus christ on a moped there were lots of dust, haha.
If haven't done this already, here's a good way to clean it out:
1. With the PC off and case popped, ground yourself (by touching the case on a metal part, static electricity is a massive killer of pcs) pick out any sizeable dust bunnies with your fingers. Try not to touch parts whenever possible, as the oils from your fingers can leave a residue.
2. Grab a can of compressed air, and with short bursts 6 inches from the components keeping the can upright, blow out any and all dust you can. The short bursts are important, as the air coming out can condense on the parts if you blast for too long. And that's BAD. Using the little straw nozzle that comes with the can works best.
3. Make sure you stick a pen or something in each fan as you clean it to keep it from moving, so the air does not try to blow them in the wrong direction or too fast (which can hurt the bearings.)
Also, it's best to keep the area around the pc free and clear, so take anything stacked on top of the pc away, make sure it has a good airspace around the sides, and if possible, raise the tower off the floor with a stand or something (slows down future dust collection.)
With the computer running, take a visual look and make sure all of your fans are running, including the one inside the power supply.
Get a utility like SpeedFan:
http://www.almico.com/sfdownload.phpInstall and run it taking a look at your temps. Let us know what you see, especially if/when it starts slowing down