I thought the best way to use an ATX supply was to set it up to always be on, and then switch the AC input voltage on and off.
If you can just use a momentary switch, I can't imagine there'd be a problem with a quick flick on-then-off of the power switch, or maybe even there's a momentary style switch with a spring that would fit in place of the original one.
As for which lines, well, I'd say you need a voltmeter to figure out the +5V, +12V, -12V, and ground wires (though they probably are color coded, yellow is usually +12, red +5, and black ground. Blue for -12v? Who knows.)
