I'm using the stock cooler with my 640 and no overclock. Using Speedfan, coretemp, asus probe, BIOS to check temps.
My prime95 load temp ~55C max at the CPU, 60C on cores, and my cpu fan doesn't even ramp up from 3375 RPM, it can do like 5000+ I believe.
I just put 2 new enermax ucma12 120mm case fans in. I have coolermaster Elite 310 case, PSU top, 120mm rear exhaust, 120mm front intake, side vent with no fan.
Gaming with my HIS hd5770 at stock speed it will hit 50C max, and again the CPU fan stays at 3375 which is almost dead silent. I have Cool'n'Quiet enabled in the BIOS, but I have Windows 7 set to not use it. Q'fan is disabled in BIOS for cpu and case fans. Asus 790x mobo.
Also, sometimes these processor read wrong temps, theres a 15C offset(sometimes 10 like I think mine is) that needs to be added, so loneninja's temps are more likely 30C on the cores.
I idle at ~35C with ambient around 22C. (I've checked the mobo at idle with the side door off, and it gets to within 2C of my ambient thermometer, so I'd say the sensors have reasonable accuracy)
Also, with the new fans installed my mobo temp rarely goes above ~30C while gaming or even Prime95 loaded, and I think the mobo temp is factored in controlling the CPU fan speed, so thats why my fan never ramps up anymore. Or maybe somethings wrong with it?
The fans were about 15 bucks shipped each, but well worth it for a quieter and cooler PC. Just goes to show, that a fancy shmancy aftermarket cpu cooler won't do much good if your Case's airflow isn't adequate.
*Warning: Meandering rant with pseudo-scientific reasoning ahead.* They are just voltage differences that are measured by the mobo and reported. There are lots of ways they can be inaccurate, so you need to use them as ballpark +/- 5C from actual is reasonable. Sometimes the software that is reading the temps is also wrong, so here's my thinking on it:
My "CPU temp" at idle is around 33C right now, and my Core temp is reported as 23C. Now without any load, shouldn't those be identical, theres just some metal and circuit board separating where those temps are being pulled from and everything should be at about the same temperature there. So I think I have an offset equivalent to 10C on my core temp. I could be wrong, but everything I'm reading online from other AMD owners says core temps report anywhere from 5-10C higher than CPU especially under load, so I might even have a 15C offset, but that would give me a 65C under prime95 load and that's not shabby with a stock HSF.
Any questions or thoughts are welcome.