Timed LED circuit with ATX PSU, help
The ATX power supply sounds good. You won't need regulators for the LEDs. The ATX power supply should be good enough. However, you will need current limiting resistors. As for the timing circuit, I believe you have overthought it. I would simply use a plug in timer like that of the local hardware store. For example:

http://www.amazon.com/Best-Sellers-Home-Improvement-Plug-Timers/zgbs/hi/6291363011.

For about $10, you can set the on time and off time. All you have to do is plug the power supply into the timer and the timer into the mains.

Better yet, I would look at getting a 12V wall wart power supply for the lights. The ATX is probably cheaper if you have one lying around, but the wall wart would be much more compact.