PS - I missed the 2 previous posts by the time I finished typing, also I assumed you were getting 4 7850s which would be jsut faster than 2 7950s or 3 7870s.  As you are only getting 3 7850s they will fit on one motherboard, but you might as well just get 2 7850s, which would be cheaper and produce the same mhash, or 2 7950s for a similar cost ($300 for a gigabyte I picked up on friday - and we don't even get rebates in Australia!) and produce the equivieltn of 4 of those 7950s with better power consumption and less heat generation, and way better airflow/heat disipation.