Hi, I bought a Nvidia 9800GT (Green edition) video card. When I put it in the monitor never displays anything.?

It's almost certainly a power issue. Not only is the raw wattage and possibly not having a PCI-E connector a problem, but you probably don't have enough amperage on the power supply either. I've never seen a 250W power supply that had more then 10-15 amps on it, and you probably need about 18 or more to run that card.