In that motherboard. for dual channel you must put in slots 1 and 3(Channel A), 2 and 4(Channel B)
Install the RAM like this
Slot 1 1 Gig
Slot 2 512 Meg
Slot 3 1 Gig
Slot 4 512 Meg
But, I have the same setup in the kids computer 2 x 1gig and 2 x 512 = 3 gig in Dual Channel.
1Haplo