650w is overkill rather than under... even with 4 drives i doubt your psu would ever draw more than 400w unless you have a highend video card setup. I doubt its the ram, if it was the computer would powerup as normal but fail the post test.

My suspicions in order would be

1. PSU(very likely)
2. CPU
3. Motherboard (pretty unlikely)

I worked in a computer store for 3 years and im certain it will be one of those 3