Have you tried breadboarding your system? This is accomplished by removing everything from inside the case and rebuilding outside the case. What you're experiencing could be either of these reasons:
1. Case short
2. Failing integrated chipset
3. Failing GPU
4. Bad RAM
5. Bad PSU
6. A shorted peripheral device
Granted that you've checked the PSU voltages, you didn't post what the results of the test were. Also, simply reseating RAM doesn't always mean that the RAM is good. I suggest you do a breadboard and add only one component at a time while testing. If you still don't get a display, see if you can either borrow parts to check in your system, or use your current parts in another system. When you find the failed part, replace.