I got my Dual 1.8 last week. Since I had purchased my single 1.8ghz less than 10 days before, I decided to pay the restocking fee and upgrade to the Dually.
Since I made the decision to switch at the last minute, I didn't bother to make a mental note of the speed of the single 1.8 to compare with the dually.
The interior is more impressive. The single 1.8 had this big empty space that is now filled with a 2nd heat sink.
Both units were very quiet. Not like the Windows box I built. When I built a Windowsbox, I wasn't sure about cooling so I put a fan in every open fan position. If I recall, it came to 4 90cm fans and one huge one.
It sounded like I was sitting next to a vacuum cleaner. I had to disconnect the big fan.