Then you're limited to getting a mid-range card, since you have only a 250W PSU in your PC, and getting a powerful card like the 9600GT will need you to get a new PSU as well. I'm not sure if you'll be able to fit a regular ATX PSU in there, so you'll have to measure the current PSU's dimensions and see if you can get a new PSU with the same or smaller dimensions. For the 9600GT\GSO or the 8800GT, you'd need a PSU with at least 26A on the +12V rails combined.
And no, Crossfire doesn't work with an onboard video chip, besides the fact that your motherboard doesn't support Crossfire at all.