I have the same mobo but different ram,however you can go into the bios:
Chipset configuration/Dram frequency/enter/user the cursor and select 400 if showing.
Im guessing its showing 400/333/266
What damage this could do,im unsure as its normally backward compatible and would normally run at the slower speed if mixed ram.
Im assuming your bios has detected the ram and assigned the speed under the auto configuration.