Monitor sleeps when connected to POV GeForce GTS450

I recently bought a POV GeForce GTS 450 2GB graphic card. But when I connect my monitor's cable to the card slot and turn on the system, it shows " Entering Power Saving Mode ". But the system unit turns on and works properly. The windows logon music is heard too. Monitor just goes to standby mode. But system works. My monitor is DELL 21 inch LED. I had a 250w psu. Is it because of that...?? PLS HELP. Waiting for ur suggestions.