Optiplex has a top cover that slides off. Take it off, leave it off and mount a card and go to town. You can pickup up some server PSU's that will run 3 cards on 3 PC's. Make sure you only use the server PSU's for the 12V rails.
Can you send me a link of example PSU? Have you tried mining like this? Thanks for the reply, most useful one yet.
Sure just search eBay. There are used server PSU's all over the place 1100 to 1400 watts for less than $50