Maybe you have a wire in the wrong hole in its plug somewhere. Try unplugging all of the four-pin drive plugs from the drives, lights, fans and anything else (and unplug the drives' data cables too). It would probably be smart to leave them that way until you're sure what's the matter here, by the way.
If the video card requires a power cable, check that it's got the red (5-volt) and yellow (12-volt) wires in the correct holes in the plug (and check the rest of the plugs too, of course).
Now, your board has a standby power LED in the center of its bottom edge, correct? That's lit when the board has power. If you turn the PSU off, then turn it on again, that LED lights up. Then you try to actually start the system and it dies. Does that light stay lit, or does it go out? I'm trying to determine whether your power supply is actually turning OFF like it's tripped an internal circuit breaker, or if it's the motherboard pouting about something. So tell me if that LED stays on after the board has died. Watch it for about 15 seconds, since the board's capacitors could power it for a while.