You can't overclock a "motherboard" per se, because a "motherboard" is not just one thing. You CAN overclock components integrated/plugged into the motherboard, like what Drew455 mentioned. Overclocking the different components is usually the only way to overclock the CPU.
Your Athlon64 X2 6000 is pretty much running as fast as its architecture is going to allow. If you're lucky, you might be able to get a couple hundred MHz with good air cooler.
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