RAM and m motherboard

Hey guys i have a m4a78 main board pro and originaly there was two 2gb sticks i have now brought another 2gb ddr2 sticks and for some reason the new ones work but only 1 of the old ones do. If i put both old ones in the computer doesnt show on screen. On the mother board the ram slots are 2 yellow and 2 black slots if this helps.

Thanks luke.