The combination of two Radeon HD 6870 cards in CrossFire cuts loose an interesting beast. Being priced at $480, it immediately feels like a big purchase. On the other hand, it offers more price/performance than the GeForce GTX 480, Radeon HD 5870 or Radeon HD 5970, the trio of single graphics cards closest to this - and is faster than all of them. Especially against the HD 5970 we see quite a performance difference. But then, it is the maximum configuration of the HD 6870, you cannot add a third of fourth card later, if you need more performance.
Another important point here is that AMD has more cards coming which will probably put the performance of this CrossFire setup into a single card, and you will be able to CrossFire two, three or four of them. The HD 6870 CrossFire might work out for you if you have just enough dough for one card now, and the scope to add a second card later. So the bottom line is that the HD 6870 CrossFire setup is more of academic value than something you should look at purchasing without knowing all your options that will be available in the next months.