I have a Dimension 4550 1.8GHz, 640MB, NVidia GeForce4 MX420 video card.  Many games exhibit stop-action video, especially noticeable in the storyline (basically mini-movie) parts.  I recently upgraded memory from 256 to 640 MB without any improvement.

 

What causes the jerky video?  If a new video card is needed, how do I find out which ones are compatible? 

 

Any advice greatly appreciated.  Thanks.