If you OC like a banshee, the s939 opteron 170 is also a great choice.
For lower OCs and no OC setups, the SLI 7800s and dual cores are hard to beat. Well except for the pricetag. All a little rich for my blood.
Thats a HUGE pricetag
I would say ge the 3800+ and OC....you are going to have to OC to get the cpu at its best and they are all the same chip and OC to the same level.
That is going to be a beast machine....make sure you have good case fans, a good power supply, and some decent RAM that will OC....like Ballistx (2x1GB)