if you use one ddr memory with 667 and the other at 1066, the 1066 will just function at 667mhz regardless kung saang slot mo ikabit
dual channel simply means you get a bit of an improvement by using two memory sticks together, as compared kung isang stick lang or dalawa pero hindi dun sa dual channel slot
its not that big of an improvement but its improvement nonetheless. also, while it is recommended that you use two identical sticks, its not necessary. you can still dual channel even if the sticks you use are different brands, they just need to have same size (i.e. 2GB) and interface (i.e. DDR3). kahit different speed pwede rin mag dual channel, the memory will assume the slower speed of the two