I was looking at video scalers (convert composite and/or svideo to hd15 or dvi-d) in a catalog earlier. They're around US$1700 or so, and I'm not sure that these were even of a very good brand, either. Home theater freaks probably spend 3 times that much on video scalers for better quality.
Your best bet is to get something like the ATI Radeon 9000 Pro which offers excellent 2D image quality (not as good as Matrox, but the ATI has much better gaming performance than a Matrox G450 could ever hope to have), and dual display outputs for dual desktop setups. It also has TV-out, AFAIK, if you need that.