if you must know, enabling dual channel, you need identical RAM, same speed, same density, same RAM chip configuration , and of course the board that support dual channel

DDR2 667 + DDR2 800, no... it won't enabling the dual channel feature, as i said earlier, it must be an identical RAM module... (yes of course you can also use other brand (Kingston + Apacer)) or any brand mixed, as long as both RAM modules same RAM chip configuration, density and so on...