It's almost always a graphics driver problem. Try http://wiredforwar.org/topic/167/announcing-community-patch-1-5-godsend or Voodoo patch (search the forums).
@pianist88 You might get better performance if you install dgVoodoo 2.
Some parts of the game may still run a bit slow at times - I think maps with day/night cycle might suffer from some weird performance issues on modern computers.
Sorry for the late reply.