Its surprising that the  G3900 does 5 cards at all. That CPU is very weak (only two threads) and due to Nvidia high CPU load with OpenCL and the work to be done sha-ing the results it is great to know it does 5 of these. Optiminer somehow found a better solution to handle the Nvidia OpenCL CPU load. I will try if I also can do better because the current situation where hash-rate drops from time to time and more cards make the system freeze is well ... undesirable ^^
Maybe thinking one month ahead I think I would like to implement a switch shifting the sha256 calculation to GPU as well ... then a G3900 should be able to power 6-8 cards when the tiny processor does not have to do the target calculation itself...