I originally had an ati 1950xt with 256mb ram with 19 inch monitor at 1280 x 1024. However I upgraded my monitor to a 23 inch which i run at 1900 x 1080 and that video card could just not handle it. So I bought a 9800 gtx+ 512 off of newegg for 125 shipped. It runs STO great!
The 1950 is a higher class of card than the 8400. I don't follow ATI much but if I recall correctly the x1950 is roughly in the same class as the Nvidia GTS 250. The 8400 is comparatively very low end.