If you'll allow me to quote Gordon Mah Ung, from a popular North American PC mag, in his response to a question regarding performance with respect to your particular question:
"Most CPUs perform best when their frontside buses and RAM are running at the same speed. If you were to run the Athlon XP with 333MHz frontside bus and 400MHz DDR, the performance would be worse than running it at 333MHz/333MHz. For the record, we used Corsair DDR RAM rated to run in excess of 400MHz; we just had it clocked at 333MHz with CL2. I'm sure Intel would have liked for us to run it at 400MHz, but AMD would likely have been miffed."
I hope this proves useful to you.
PS - IMO, Gordon is a fountain of knowledge; and without transgressing by advertising the magazine he writes for, I can only say that he and his compatriots are top-notch.
Mick
Complacency is the rigor mortis of the soul.