tiny voices said:
Not even close to true. http://www.anandtech.com/bench/Product/513?vs=549
Even with 2 4870's you won't get close to one 7850. You will use 3x the power if not more also. Don't forgot that 2 cards does not mean 2x the power. Not how crossfire/SLI works.
Yes, I know how CrossFire/SLI works. I know that having two of the same card work together doesn't mean that it'll perform twice as fast as only having one of that card.
That comparison is between one HD 4870, not an HD 4870X2 dual-GPU card. It's only natural that a single HD 4870 card would perform like that compared to a single HD 7850.