No, you don't adjust it. You're supposed to run DDR400 RAM with that CPU. Any attempt to make the RAM you have run the right speed for the CPU will mean a drastic overclock for your RAM, from 166MHz (DDR333) to 200MHz (DDR400), a 20% RAM overclock.
And your board might not even support manual adjustment. But if it did, using the higher setting could cause severe problems, including "no boot" or stability issues.
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