other possibilities

- dead video controller (I think you checked for that already)
- dead motherboard. (some power components?) some things might be
powering up, but not everything on the motherboard. Since you've
tested a different PS, its probably the motherboard.

- ** check for shorts between motherboard and the case body. a loose screw,
nut or something metallic between the board and the case.

- make sure the reset or power key is not stuck.