Actually, quad fire will not beat the 7970. If 2x 6870 outperforms the 7970, how will 4 5770s not? Think about it. :P
Btw, I know it was a hyperbole. :P
Not at higher eyefinity res where you need that 3GB vram.
Also, xfire 6870 does not beat 7970. 7970 is almost about 6990 or GTX 590. Thats dual 6970's or GTX 570s
And I know 6770 xfire is weaker than 6970 so 7970 will beat quadfire. And after 2 cards, the performance increase added per card drops. A third 6870 only adds 20% increase to performance.