Yeah, if you have the Antec True380's fan-monitoring wire plugged into a fan header on the motherboard... don't do that. Also, your system really calls for at least PC2700 memory so it can be run synchronously to the CPU's bus. Confirm that the Radeon 9800XT does have its auxiliary power pigtail plugged into the power supply, and double-check that your heatsink is installed
this way.
Is it using the stock heatsink or aftermarket, by the way? What kind of thermal-interface material (grease/patch)?
If it works outside the case on cardboard, then the symptoms may be indicating that it was grounding out somewhere, like
johnjkr1 said. One of the possibilities is that your case's front-USB wiring was plugged in incorrectly, or that the EMI springs on the I/O shield were getting into the USB or network jacks on the rear and grounding out in there. Any of that sound like a possibility?
The board has speech diagnostics, but I don't know if they'll be useful or not. Plug headphones or powered speakers into the green audio-out jack on the rear of the board and see what it has to say.