ATi is currently the best in single-card performance(unless you get a GX2, but that's 'cheating'), though nVidia seems to be better in the bang/buck area. But I'd wait until someone else verifies that because I'm not sure on all the little details of that.
Anyway, what I do know is that you can't run an ATi and nVidia card at the same time, their drivers try to kill each other. Whether that's true with chipsets as well, I have no idea; you might just need to uninstall some driver that came with it, you might not need to do anything, or there might not be anything that anyone in the world can do for it.