We could use one large memory module (often called sticks) with all of the memory we need. It is, however, in most configurations, advantageous to run two smaller memory sticks in dual channel mode (e.g.: two 1G modules instead of one 2G stick). The concept is to balance the bandwidth (amount of data) coming from and going to the memory and CPU. The performance increase is largely tied to the chipset you are using. For more on chipsets, checkout other articles in this this series. Right now we are going to explain how to install memory in dual channel configuration.
To do this, you should place two identical sticks in two slots on the motherboard. Ideally, these memory modules are manufactured, tested, packaged and sold as pairs for the purpose. You can make a pair from sticks bought individually; but make sure they have the same size, speed, and latencies. Exactly which slots to use depends on your motherboard.
Our ASUS P5E-VM DO has a fairly common set up. We need to have a stick in slot A1 or A2, and the other stick should go in the equivalent B slot. Choose the pair you prefer based on which are easiest to get at. The picture shows, with yellow rectangles, which slots will be used.
Note that cutting-edge DDR3 memory, with the appropriate motherboard, can be set to run in triple channel mode.