Could be a million things honestly. The best plan of action is to eliminate as many variables as possible. Try just hooking up the PSU, mobo, CPU, and one stick of memory. Remove everything else and see if you get a beep code. If not, you may have to remove your mobo completely from the case to eliminate the possibility that it is shorting on the case itself (this is called breadboarding). I assume you installed your standoffs in your case right???