I have the Asus M3N78-VM pretty solid board had no problems with it
It is solid. Better stick to it for now and save up for an AM3 motherboard + DDR3 RAM.
We don't know what is the complications in a hybrid RAM system and you may be in for trouble if you go that route.
And honestly, I do not see how you will need more than 4GB of RAM but I you feel you need more now, why don't you spend the money on additional 4 GB of RAM and you will have 8GB of RAM in a solid board instead of spending it on a hybrid RAM motherboard.