With a stock cooler for a CPU, the overclocking rate is limited to the heat produced at the peak of the CPU usage. In order to run your CPU at highest bus speed, you are required lower your CPU temperature by using better CPU cooler. On the other hand, to test your system stability, you must use stability tester such as prime95 and orthos to run stress test at least 2hrs, 24hrs stress test is recommended. Moreover, you should try to lower the CPU vcore in order to lower the CPU temperature. However, if the vcore is set below the CPU stable voltage at that speed, it will results in error or rounding problem. Testing for stability consumes a lot of your time, but if you enjoy the process, you won’t mind this.

After another few days testing, my AthlonII x4 640 C3 is able to run at 250×13=3250 @1.325V. It can’t go further more, due to the limit on a stock cooler, using stock cooler your CPU temp may be around 66 degree. But if you use an after market cooler, the CPU temp may down to 56 degree, approximately 10 degree depends on the cooler materials. It is not worth to do so, as the increased performance was not higher than using higher level CPU, but you can enjoy faster response after overclocking bus speed and lower fan noise depends on the CPU cooler.

However, after my testing the most efficient speed is 220×15=3300 @1.325V, as it has lower CPU temperature but close performance than 240×14=3360 @1.350V and 250×13=3250 @1.325V. I am still sucked on choosing which speed to operate for my daily uses, but I love to use higher bus speed. Because when CPU is idle, it will lower the multiplier to x4, which means 250×4=1000, 200Mhz faster than default. Then it will response faster in low CPU usage, I guess…