everything worked perfectly in the end, I just bought a new 550W PSU, and tried it with AND without changing the CPU & RAM voltages, and it is true that with the system default settings, it all is more stable (plus: with the voltage modifications , my CPU got clocked down to 1250MHz!).
Now, only one new problem occured, which is quite minor:
when I restart my system, the bios freezes right between Primary & sec. Masters & slave detection and CPU clock detection, for a couple of minutes (however, it goes VERY fast when I just boot it up, without restarting from the OS) :beard: