rtfm

[shadow spoke]

Make sure those ram sticks are in the right slots. There is a rank order in which you have to populate them for dual channel memory to work. If you do it wrong, you'll get half the performance out of your ram (but you'll still get full ram capacity)

Every mobo can be different so the only way to know is to read the manual. But I can safely guess the two sticks are in the wrong slot.