You probably need to go in to the bios and set the ram to run at 400 mhz and you might also have to set the timings manually.
The timing settings you are looking for are named CAS, tRCD (RAS to CAS Delay), tRPD (Row Precharge Time), tRAS (Min RAS Active Time) and should have numbers beside them. Set CAS to 3, tRCD to 4, tRPD to 4 and tRAS to 8.
These will be safe settings. If you know your memory timings, something like 2.5-3-3-7 you can set them as that. Just remember that you have to set them to the slowest stick ie the highest numbers.