You should see a mention about memory channel configuration in bold characters during POST (something like this : <b>Dual Channel Mode</b> or <b>Single Channel Mode</b>). Usually the board auto-detects the configuration. The Asus A7N8X DX has 3 memory slots. A slot 1-3 or slot 2-3 configuration enables Dual Channel Mode automatically. If you wish to fill your memory slots and still take advantage of the Dual Channel Mode, you need to insert the total amount of RAM of slots 1 and 2 into slot 3 (i.e., slot1 = 256Mb, slot2 = 256Mb, slot3 = 512Mb) preferably with similar timings.
<font color=red>That was my two cents!</font color=red>
<font color=blue>Now I'm broke!</font color=blue>