PSU/graffix card

I wouldn't recommend it, but then again video card manufacturers have a tendency to put higher energy requirements on video cards to avoid being blamed by people who have a crappy psu's. I highly doubt that a 250 w psu could handle the card, and the potential of frying your components is just not worth the risk. Instead I would recommend buying a high quality psu and replacing the one you have. It's a fairly simple process and you should go to newegg.com to find one that fits your price range and requirements.

card geforce 8400 gs 512mb 64 bit/ psu not sure but the sticker inside said 250w

We still need more details on that PSU, the most important being the 12V Amp rating.

You could go here: http://extreme.outervision.com/psucalculatorlite.jsp and fill in all your system information, selecting the 8400GS in the video card option. It'll give you a rough estimate as to your power needs.

You can also find the 8400GS on this chart: http://mark.zoomcities.com/images/ [...] rtby3d.png and compare it to your current video card.