Suggestions? Just the standard steps - check all cables & connections, reseat expansion cards, double check any jumpers on mobo & drives, take out all non-essential hardware (modem, NIC, sound card, etc.) and reboot. You know, that stuff. And the things you're doing, like checking as much hardware in other machines as you can.
Try a different processor if you can, or see if reseating your T-bird 1.4 will help, or try your T-bird 1.4 in another machine. Looks like good quality parts, but can't say they won't go bad. Does your rig at least POST? If it doesn't even make it to the BIOS (or you can't see it anyway) then maybe try clearing the CMOS? Gotta be the proc or the mobo at this point (if your GF2 Pro works in another machine).