Bear in mind that for dual channel to work your need to use two identical modules.
Both sticks mush have the same chip density, be single/double sided, and run at the same latencies (if the latencies aren't the same your bandwidth may drop as often your mobo will underclock the slower chip to 166 to get the same timings).
Its preferable to use the same part from the same manufacturer, this is why dual channel "kits" exist.
Intel core I7 920
6GB OCZ platinum 1600
Samsung 1TB F1 Spinpoint
Samsung SM2443BW 24"