well you can't run crossfire on that, only SLI.
But like I said, you should stick with a single card setup. It's less hassle as 2 cards don't always function properly together, not to mention twice the power required and twice the heat output.
There are going to be some superclocked 4870s out soon, you might want to wait for those as they will be relatively inexpensive compared to the GTX 280 and should be far faster.