RAM's fine, its DDR2 so you have to double the rate.
Think of it as a motorway with two lanes, you have 400mhz going in one way, 400mhz the other therefore 800mhz total.
Also, remember that FSB speeds need to be divided by 4 to get the actual front side bus speed.
[EDIT]Duvet77 beat me to it![/EDIT]