Need help adding 5th GPU on Gigabyte 990 fx-a UD7 mobo

Hey there. First time poster here. My friend refered me to this site. I started mining (scrypt) not to long ago and built my first decent rig. I cannot seem to get the 5 card running. They say on their website that it can support 4 GPU's but it has 6 16x PCI-e slots. I can use any of those slots to run the 4 cards on 16x to 16x powered risers with no hiccups. Can someone please point me into the right direction. Here is a list of my specs

2 - 850watt Corsair RM850 PSU
Gigabyte 990 FX-A UD7 mobo
AMD Athlon II X2 270 Regor 3.4 GHz 2x1 MB L2 Cache Socket AM3 65W Dual-Core Desktop
4- GIGABYTE GV-R929OC-4GD Radeon R9 290 graphics cards
Corsair Vengeance 16GB (2x8GB) DDR3 1600 MHz
Windows 8.1
CGminer and CGwatcher

Thank you in advance to anybody with some input.