I have a system with a single 7850, and am possibly going to buy another to run CrossFire. The problem, however, is that the second PCIe slot (Supposedly also x16) appears to have a "x4 mode". I'm wondering if there will still be a significant increase in performance if ever I run CrossFire with a second 7850, and how badly will it compare to running CrossFire on two x16 PCIe slots?
You will still get an increase in performance if you crossfire with another 780 but noy as much as running both cards on two x16 pcie slots. The performance difference will probably be 10-15%.(my guess)
I know it's said that anything under 8x will be bottlenecked for a GPU, but when I did my two 4870s 16x/4x I still saw a worthy improvement (50-60% roughly). I have an ASUS M4A78 Plus. I would still recommend 8x or 16x in both slots, but it can be a cheap upgrade now, then you can get a new mobo later.