800W psu is unlikely(but not impossible) to be a problem then unless its got a fault.
BTW, any chance you can borrow an nvidia card from someone that needs the 'extra' power line/s?

Also, when i said to try with another monitor on your PC, I wasnt siggesting yours is faulty,
I was looking for the posssibility that the new card "doesnt like" your monitor.

See if you can look up the motherboard bios details on the net. Your motherboard
may need a bios update for that video card...