check the settings on the monitors' menu for the hz setting, set it to the lowest setting and reboot the pc, if still nothing happens then your video card is most likely what we in the industry refer to as F*cked but good thing is prices on good video cards are at an all time low..
I had a generic ati once that caused similar problems, i just went to ati's website and downloaded their drivers instead of the ones visiontek updated, totally solved my problem.. dunno if it helps but its worth a shot..
(edit) also, did you check your bios setting to make sure your card slot didn't reset when you installed the new video card?