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.
AMD Athlon XP 2000+
700 odd mb DDR RAM
60 GB 7200rpm Maxtor Harddrive
10/100 Local Area Network
nVidia 128mb Geforce TI 200
Abit AT7-MAX2 Motherboard
I would suggest zootjeff's 220 watt PSU. You CPU requires 5A+ alone on the 12V rail. Not sure if you are using a 3.5" or 2.5" drive, but assuming 3.5" which will require another 2A. DVD drive internal will also require more juice on the 12V rail, as well as fans. You also need some room for your monitor if you will power it off the PSU. If those are all desktop components, the Opus 150 watt will not be enough. You could get the Opus and get a Carnetix 1260 or 1290 to power your drives/screen. I think zootjeff's PSU is your cheapest solution.