Give us some more specs as to what type of processor, hard drives (physical size and speed), graphics card, usb devices, optical drives, etc.
It's not as simple as knowing how many watts your system uses. There are individual lines on a power supply, 3.3V, 5V, and 12V being most important. Unfortunately just because your temporary PSU is running right now, doesn't mean you aren't pushing it over it's limit and asking for a failure. Depending on your needs, you may want to go with an Opus 150W or a M1-ATX paired with a 1260 or 1290 to power your drives and USB devices. If you give us some more details it may turn out you only need an m1-atx.