New Case - Computer turns on once, wont repeat

I just got a new case delivered today and so I transferred all my existing stuff from the old case to this one. I went to turn it on and it powered on with no problems except for one, when I booted into Windows I was warned that I had forgotten to connect a power adapter for my PCI-E card, and that it would run in a limited capacity until I did so. I continued to keep the system on for another 5-10 minutes while I checked my email and other little stuff.

After those approximately 10 minutes I powered off, opened the case and made the PCI-E to Power Supply connection, put the case back together and when I went to turn on the system nothing happened (No POST, no fans turned on, including the PSU's fan - The system was literally dead)

I disconnected the video card to see if that would help and nothing happened still. So I began disconnecting everything, to the point where all I have connected now is a CPU and a hard drive (completely removed the video card for testing purposes) and yet still nothing will turn on.

I'm pretty sure it's not a power supply issue... When I connect the huge 12-pin power cord for the mobo and then connected the power supply to the wall the green indicator light on the motherboard comes on, when I disconnect from the wall it goes off.

I've also tried hooking up another PSU and the same thing occurs (Green light, but nothing powers on)

Also, to get it out of the way, I did toggle the on/off switch on the back of both PSU's, and they've all been set for the correct voltage. I also tried resetting the CMOS by removing the jumper and waiting 5 seconds, then replacing it, and that didn't do anything either.

My specs are:
Ultra 550W ATX PSU (Also tried an Antec 500W SmartPower 2.0 PSU)
Asus A8N-SLI Premium Motherboard
AMD Athlon 64 X2 4200+
2GB Corsair XMS RAM
BFGTech Geforce 9600GT
1x CD-RW and 1x DVD-RW
3x WD 640GB SATA 2 Drives
1x Seagate 400GB IDE Drive

The case is an NZXT Tempest, it includes
3x 120mm Fans
2x 140mm Fans

Any ideas what the problem is? I'm afraid that the motherboard may have gone out for some reason, as like I said before I think the PSU is just fine. That, or I think the case may just be bad. I once had a computer where the case's power button one day up and refused to switch on the system (switching everything to a new case fixed it.) Furthermore, I question the quality of this particular case sent to me, since the wire that enables the HDD LED Activity light was slightly frayed upon receiving the case.

Jam it back in, in the dark.

Trying another motherboard in that case will probably answer most of your questions...

There's nowhere I can't reach.

I lied, I decided to hook up the bare essentials again and give it one more shot. I noticed the power to the CPU wasn't fit in a snug manner, after I reseated it the system powers on fine.

This thing is sticky, and I don't like it. I don't appreciate it.