If it is a standard ATX power supply, what's the power supply rating.
Just a thought -- You can probaby sell a whole working computer of that vintage is for more than youy can get for its parts, used so you may want to do that and build a new system. That way, all the other parts will be of current design, as well, and they will work well together. The other side of that coin is, trying to salvage parts from your old system will mean you're trying to include older stuff with your new setup, and they may not work as well as you hope.
Good luck.
