Computer won't boot - weird issue

Hey all. I have been coming to this forum for a while and have always found it very helpful, so I'm appealing to the collective wealth of knowledge for help with an issue that has me thoroughly stumped.

I built a new PC back in December that had been running wonderfully until tonight. My build is as follows:

ASUS P8P67 Rev 3.1
i5 2500k overclocked to 4.5 GHz
Coolermaster Hypercooler 212+
Sapphire 6950 2GB
Seasonic X650 PSU
g.Skill RAM 2x4GB
Seagate 1TB HD

My computer was running perfectly well until tonight, when I installed the Tribes Ascend beta and started playing for just a little while (probably unrelated to the issue, but just throwing it out there for completeness of the story). I alt-tabbed, then tabbed back into the game, and the graphics were all messed up.. I didn't think anything of this, since it's just a beta and plenty of games don't handle alt-tabbing correctly. Again, all of this is probably not germane to the actual issue, but I'm trying to be thorough.

Then, with the game still up, my PC just shut off. I heard a sound like when you turn a radio off (if that makes sense) and there was blackness. Now, it won't boot at all. Nothing I do makes it boot. I've removed each stick of RAM (and both sticks of RAM), unplugged everything from the motherboard, etc. The green light on my motherboard is always on, though - as long as the cord is plugged in and the PSU switched on, of course.

Now this is really weird: the only way to get any response from my computer (other than the green light) is to unplug everything from the motherboard except the 24-pin connector from the PSU. Then, the power light on my case comes on, and the case fan (which is the only other thing plugged into the motherboard) spins up. This will not work if the 8-pin connector is connected as well. And if the 24-pin connector is unplugged but the 8-pin one is connected, nothing happens then either. Note that I don't think my PSU fan comes on, regardless of what I do. And if I so much as plug in the HD into one of the SATA slots, I get nothing.

I'm thinking I've blown either my PSU or my Motherboard. I'm hoping it's the PSU, which would be ironic given the number of posts in this thread commending me for spending the most money on a good, reliable PSU - but at least that would be fairly easy to RMA and replace. Can anyone help me figure out which one it is? Is there another test I can do to determine which one is broken? Or could it be something else?

Any and all help is appreciated. Thanks guys.

Use a multimeter to check the voltage on each rail.

Try a different power supply unit.

The most foolproof method would be to try with a different motherboard or PSU. Do you have access to a friend's build?