I still suspect that you have an x16 slot in there, despite the specs. I don't think Dell would want to deal with a custom x1 video card, and one is available in custom configuration. Best way to find out is to pop the side, maybe take a picture of it so we can look at it.
You're not going to get much better graphics; DVI connections are a little better than VGA, but it's not going to be eye-poppingly different. Blu-ray video acceleration you'll get, but I don't think you have a player in there. I'm not sure if you'll get any acceleration on DVD video.
