Ok, make sure that you have them in the right slots is the first order of business.
I'm sure that the BIOS gives you option to either have it automatically set it to dual channel or manually set it. If you set it to auto, go back and make sure it is running in dual channel.
Ok, someone set me straight here. Do you want the sticks to be in the same bank/channel or should they be in separate channels? I always seem to get this wrong. I probably have it running wrong in my mobo right now because of this.