Funny. It must be that stock heatsink. The one I mentioned that I used keeps my 670 a lot cooler. I stress test the CPU with both threads at 100% load for about an hour straight and barely heard my CPU fan ramp up. Maybe you used too much thermal paste? Too much actually has an opposite affect.
I did add a 92mm fan to the front where the hard drive cages are and put the hard drive and wired it to a 5v on the molex connector to draw in air.