Start with minimal items installed, 1 stick of ram, video card, cpu, heatsink and fan. No hard drive, no other drives, Regular keyboard attached but no mouse. See what happens. If you still get the error, try a different ram stick and after all the ram is tried, try the other GPU.
Also, double check and make sure graphics card is seated properly and the 6 pin power connector is plugged into the card properly. There might also be a 4 pin square plug that needs to be plugged into the motherboard as well.