Stick with the 250. You will get a bit better performance out of the gx2 at lower resolutions, but if you're playing at 1920x1080, the memory on the gx2 will get in the way - its 1gb RAM only functions at 512 mb (in a SLI/xfire configuration, video data is duplicated in the RAM for both cards. a gx2 is SLI on one card.)
effectively, the gx2 has half the RAM of the 250.
Also, I don't know what the most current drivers are that the 9800's can use, but if they're older than 2 years, you won't be getting the benefit of Nvidia's frame metering. Frame metering spaces out the frames rendered by each GPU evenly, so you don't have runt/dropped frames. runt/dropped frames are ones that will show up in a fps benchmark but are too small to notice when playing, so you get no benefit from those frames.
On top of that, to run the gx2, you'll need at least 35a on your PSU's 12v rail.
even more, SLI scaling was poor prior to the 400 series. you'd get an extra 50% performance from adding a second GPU if you were lucky.
stick with the 250