Yes, 400MHz RAM will work just fine on your board. The only issue is that you might bottleneck your other components, which pretty much slows down your other parts to make them work with your RAM
thanks but i have one more qestion. looking arround on this forum i noticed manny people asking about " 800 mhz ram running at 400mhz " m i one of them :/ my speccy and cpu z says my ram is 400 mhz