The ram still runs at 400Mhz(DDR1) when in dual channel, it's just that when it's in Dual channel, both the sticks are used together, suppose two 512Mb sticks are put in single channel, then there's a 1024Mb ram with 64bit interface, If a file is to be written on ram, it will go to any go to any of them through the 64bit bus, but now if they are in dual channel, it will be 64+64 bit, the file will go partially in both of them, all the banks are considered to be one ram, so it will be 128bit.
not just any two slots in motherboard will run in dual channel, diff manufacturers have diff designs, for some it is adjacent slots, for some alternate, just look at the forum,if you cant find, ask here.