The MCH may have a limitation when used with 4GB DIMMs where they need to operate in single-channel mode. If you have 2GB DIMMs laying around, try those instead, and in the exact same slots. If it shows dual-channel with 2x2GB, but after reverting back to single-channel with 2x4GB, then it's an MCH limitation.
Easier method: contact the motherboard vendor (Asus) for clarification. They'll be able to help you.--
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