Well, the first thing I would do is drop to the X4 955. There isn't any real difference between the two other than a slight factory overclock and $20 bucks.
For the board, I'd defintiely get a Gigabyte GA-790XTA-UD4. It's decently priced and a great board.
I should also point out that starting a build with Crossfire isn't that great of an idea. I'd spend the same amount of money and get a HD 5850 (like in this combo with the board]. That will be powerful for the majority of games and still leave an upgrade path for you later.
You're spending less than $300 for the 5850 ($400 total for the combo with a $110 board). And no. The new Fermi cards aren't worth paying extra for when you consider that you could spend the little extra ($35 with a combo) to get the better HD 5870.