Take a look at the ATI 4850 benches, they are faster than the 9800gtx and two of these in crossfire are faster than a GTX 280. The 4870 will be out soon and two of those in crossfire are going to leave a gtx 280 in the dust. The 4870x2 will also be out a little after the release of the 4870. The 4870x2 is two 4870s on one card, so it's a single slot solution, and going on the 4850s in crossfire performance, it's a certainty the 4870x2 is going to be a lot better than a gtx 280.
So all that said, if you are willing to spend that much cash for two GTX 280s, just wait on the 4870x2 release. Buy two of those, it will destroy an SLI GTX 280 setup. And an added benefit is you can use an Intel motherboard, rather than an nforced based mb. Intel mobos are a lot better than nvidia offerings.
If you're willing to spend that much cash, you obviously want the best. Best to wait for the 4870x2 to come out, it will be a better performer than the GTX 280, and two of them will be much better than SLI 280s as well.
:beer: