May I say, a great system you have there
As for the motherboard, it depends all on what features you want.
Do you want it to be SLI, so you can add another 6800 ultra at a later date to add to performance?
What type of RAM do you want it to support? etc.
These are all questions you have to ask yourself, and there are so many companies out there, its hard for me to just choose one you might like, since the motherboard is what everything runs on.
I've got an Abit motherboard, and I think they are a good make, though theres also Gigabyte (supposed to be very good), ASUS and DFI, etc.
If the case may be that an X2 will work on a standard 939 board (I think it does), this seems like a great motherboard:
In a review, it tested the exact processor you had anyway so it seems like it does support it, and also got excellent results