I had a similar issue with a build I did back in December. I built a rig with an A4-3300 APU and 8GB of Corsair Vengeance (Low Profile) 1333mhz ram. The rig would work fine with a dedicated graphics card, just didn't work with the onboard graphics.
Try taking out one stick of ram and see if your system posts. And if you can exchange your ram, go and get it exchanged for an 8GB kit that's listed under the supported ram list
It'll save you a lot of headache.