If it was me, and it soon will be, I wouldn't do it the way described in hobbycnc. I would leave the At or ATX power supply alone and connect an old floppy drive to give a load to the switching supply. Mount the supply inside another box. Then construct linear DC to DC supplies to outputs on the outer box. This would give you a 12 volt, 9 volt, 5 volt and a 3.3 volt outputs, with the proper dc to dc supplies. And you can made an adjustable output for anything in between. Adafruit even has a kit to make an low dropout dc to dc supply. So then all you would need is a couple of panel meters and you are good to go. It looks like an hour and a half project to me.
When I do mine I will make an instructable and show what I am talking about. But there is no need to open the comp power supply. If you did it that way it would look like crap anyhow.
Sparkfun has atx supply headers in stock.