yes, it will just downclock. The only reason to buy higher rated ram is so you can OC the cpu while the ram is still being downclocked which means your ram can run a 1:1 ratio with your CPU which means technically it wont OC until after you reach the DDR 500 mark
I know how to.. Ehmem... Stop the countdown.. For WINXP.. . Or my uber smart friend said it would work, i havent had the courtesy to try it out yet. If anyone wants.. PM.