^Uh, yeah pretty much everything he just said. You are still not listening.
Dual channel mode requires 2 IDENTICAL sticks of memory per bank. 2 Identical sticks per bank. NOTHING ELSE WILL WORK IN DUAL CHANNEL MODE.
While some will say that you can successfully mix different brands, or latencies, you are asking for problems. In fact, you are beggin for problems. What you need is 2X2 sticks of paired memory, for 4 gig. Buy a 2x2 gig kit, get rid of what you have if you cannot find an identical matching stick.
Now before someone jumps on me, there were some boards way back when DDR was first being used, that say they will run a single, or 3 sticks in dual channel mode. They however did require a certain type of double sided memory stick to do this. Just covering my bases because there is an exception to every rule, and around here there seem to be people who thrive on jumping in and driving home every tiny insignificant point that really means nothing to the majority of us.
(like the guy who keeps jumping in to assert a 32 bit OS that can use more than 4 gig of memory, my god I am so tired of that one)