Havent used that board nor a Crossfire setup before, but I know you will see no performance boost. Quite possibly a decrease in performance if you tried to Crossfire on that board.
You need at least a PCI-2 8x slot to not bottleneck graphics cards, below that and they will be limited by bandwidth. By running a card in the 4x slot in tandem with the top card, it reduces both card to an effective 4x bandwidth, limiting performance in a big way.
Best case scenario you spend double for no performance boost, worst case you spend double and end up with less than a single card.
It will work, however you wont get the result you want.
Notice how SLI isn't supported by the board, only Crossfire? Thats because Nvidia states that a pre-requisite for SLI is there must be at least 8x/8x slots. AMD on the other hand is fine with allowing people to use 1x and 4x slots to run their cards in Crossfire, but if you tried you will get less than stellar performance.
And anyway, assuming your running the 5850 I can see in your sig, you would be better off just getting a better single card.