The slots are set up for dual channel. If your board is version 1.03 you may need to download bios 1007 to support it. Go to the Asus web site for details on setting it up. Dual channel is picky about the ram you use. If you already have dual channel enabled in your bios already, you may want to try disabling it, as it may not like the ram you have.
The 1007 is a good bios as it also includes support for 200Mhz which greatly improves overclocking ability. My Barton 2500+ is now 3200+ (2.2gig) thanks to this bios.