I bought a XFX 6850 for $180 in Denmark and I had one already, It's a cheap way to upgrade and get almost twice the power. A single card with the same performance would cost $400-$500 in Denmark.
It's not about comparing a cheap card to an expensive card on offer right now. Rather then spending $180 now and then get another card of the same model within the year and still be behind, it's better to use the $180 to buy the latest gen card (say a 8850) that will outperform the 2x6850's. Not sure if that makes sense, but do you get what I mean?