Open to ideas/solutions?

First, your motherboard does not have an AGP slot. It has the following expansion/card slots:

PCI-Ex16 1

PCI-E Gen2 (1x16)

PCI-Ex1 1

PCI 1

Putting everything back in the computer the way it was, when you power on, do you get any beep codes? If so, the exact pattern will tell you what error the motherboard is experiencing.

When you power on, listen carefully to the hard drive. Does it sound like it is going through the boot process or does it sound like it only does its power on test?

Given what you describe above and if there are no beep codes then my guess would be motherboard and/or CPU failure. If it was some other component, the motherboard/BIOS would give you a beep code but it doesn't seem to be getting to that. Additionally if the BIOS was initiating then the new graphics card would display the post information and the hard drive would go through the boot process even though you couldn't see anything. If you know anyone with a spare motherboard or CPU, I would test with that before purchasing anything else. If you can't do that then you have to either play guess which one or perhaps just go for a good upgrade to an AM3+ motherboard and FX processor. Test these with the old processor (the old motherboard cannot support a FX processor but an AM3+ motherboard can support the Athlon X4 processor.) If the X4 processor works with the new motherboard, then assume it was the motherboard that was bad. If not, then it was the processor and maybe the motherboard.

Pair all the way down to the basics. Disconnect everything except the processor, ram, cpu fan, PSU, and power switch. I've had a bad USB cable once that overloaded every PSU and it acted a lot like what you are describing. With just what I listed you should be able to get a boot screen. Also look over the motherboard. If something fried, the capacitors will be bloated. And reset the bios while you are at it.