I would strip it down to 4 components:
1) Power supply (you already know this one is good).
Without RAM you should get a Bios beep error code for the missing stick. If not you can surmise a bad mainboard, CPU or both.
My eMachine power supply has gone bad, actually it burned. The computer is about a year and a half old, and I have not had any problems up to this point. It was turned off whenever this happened. The original supply was a 250 and I was told it was a standard PSU. I replaced it with a 350watt universal ATX power supply and now the computer will not boot up. The universal has a 24 pin connector, but mine is only a 20 pin connector. The universal can unclip the last four pins so it will fit my motherboard. It will turn on but no signal is sent to the monitor and it will not do anything other than turn on the fans. I have been told that the motherboard is most likely bad as well, but I have been also told that the power supply must be a eMachine (bestec) in order for it to work. eMachine tech was absolutely worthless since it was out of warranty. Any help is greatly appreciated.