Not sure what you're looking at. When I query Newegg for the 512 MB HD 4670, I get five options at $79.99 and one option at $80.99.
As for your father-in-law's system, what does he use it for? What did you put in there, and what's the resolution on the HDTV? If it works for him, there's no point in spending money (if it ain't broke, don't fix it).