This hasnt been mentined yet, but to have your ram runing in dual chanel (if your motherboard indeed suports it) you need to have your two sticks is specific two slots, on my motherboard its something like this:
It has 4 slots XX and YY if i have two sticks of ram which are is lots X1 and X2 i get dual channel mode, the same result comes from ahcing them in slots Y1 and Y2, but if they are in X1 and Y1 or X2 and Y1 or X1 and Y2 they will be in single channel mode.
Now about diferent memory sticks beig used and wanting to get dual chanel mode, in that case both ram sticks have to have the same number of memory chips on them and they have to be both either one sided or double sided, if that requirement is met and you set ram timings to correspond to the stick with looser timings generaly you will get dual channel operation without problems, but as it was said by a previous poster some ram sticks cause conflics with certain motherboards, be it because of the PCB design or chip incompatibilities so it is better to use identical sticks.
Hopefuly this is helpful.
Edit: Was writing my post wehn you posted the question on how to access the Bios. Anyway on most motherboards you just ahve to hit the Del key to get into the Bios menu, but if you do not confidently know what various functions do there its best to leave them alone, you might want to read this before poking arround there
http://www.pcguide.com/ref/mbsys/bios/set/index.htm