if you got stock DDR2 800 , it will show up as 400 in a program like CPUz you gotta just multiply it x2 because its 2 channel ram.
to correct steadfast. He is right about multiplying by 2, but its not because your running dual channel memory. you multiply by 2 when your using DDR (doesnt matter if its single channel, dual channel etc.)
DDR = double data rate. This type of ram transfers data twice per clock cycle. When you read 400MHz on CPUZ that is the clock frequency of the ram, when you see it advertised as 800Mhz that is the data transfer frequency.
(400 million cycles /s) * (2 data transfers/cycle) = (800 million data transfers /s)