Got it working!
Wired up according to my previous image. I didn't use any diodes - partly because of lazyness, partly because the starter manual didn't have one listed in its wiring diagram. I hope the feedback won't cause any problems (knock on wood).
I used this relay: Radio Shack 275-241
Small sucker! I have it floating free in my case right now, but will eventually secure it. The 18 guage wires I used are more than enough to hold the thing suspended in mid air!
Anyway, I hold the AUX on my keyfob for 5 seconds, and it triggers the power switch on the motherboard for around 2 seconds. Works for turing on and off. Configured windows to hibernate when the power button is pressed. Seems to work pretty well. Thanks again for your help!