Interesting. With only 6GB, understand the desire to be fully slotted.
Not sure about the UD7, but don't think you can mix triple channel with dual channel secondary slots. Asus boards I have only support a single pair of dual channel.
If you wanted to spend $40-50, could get 3x4GB modules. $80-100 would get 3x8GB. Since it's X58, it's an investment in the future, not just the past. ;-)