Do you have a case speaker? The beep codes they give off can be a huge help in diagnosing problems, and if you don't have one you can get one pretty cheap. I've seen similar behavior to what you're describing and figured out I had a dead motherboard. I even got a power error from my speaker but tested the voltage from my psu and it checked out fine. If it appears your psu is working fine then there is a very good chance you have a dead motherboard.