Simple hack for 5V and 3.3v power rails
If your PSU can produce +12V and +5Vsb, then there is a simple way to produce the +5V and +3.3V rails.
Connect the wires from the PSU to the 20-pin part of the 24-pin ATX power connector. Use the detached 4-pin part to create a dongle PSU for +5 and +3.3...