Why are you trying to save Dell case, you can get better one for under $40 - Inspiron 580 has only one exhaust fan, no intake fan and I think most important - on/off switch, which is Dell proprietary in one box.
General cases have on/off switch with individual wires, so it is easy to connect.
I would strongly suggest changing cases. If you want save PSU, Dell is using standard ATX PSU, I think 305 watt, first it is weak for anything serious, second it doesn't have, most likely, 6 pin GPU power connector, and it has limited number of other connectors.
So, the conclusion - change case.