When you first boot the computer, the asus bios program will ask you the frequency of the cpu assuming it is set on auto. It will give you two choices, which I believe are 1.333 and 1.533. If you dont set it, it will default to the lower frequencey. Go into to your bios and choose 1.533 and u should be all set. Hope this helps.
If what jedge told you to do don't work... then I have a question. Do you have a lan card in your system? I had a Lan card in my system that changed my speed down to what it could handle. I think it was at 1050 or something and then RAM to 133 when its supposed to run at 266. Don't ask me how or why it did that but just my little 2 cents.