I'm not at all convinced you'll get that sort of spec PC for $800. The Matrox rt.x10 will cost close to that on it's own!!! It's about £500 (UKP) over here in the UK! Of course if you can find it then let us know. I might order one myself.

If you're serious about it all then change your spec to the rt.x100 card and get Adobe After Effects as well.