I'm using this combo exactly (Bt 878a, though). After RFI modding the MPACT, and with the newer versions of Dscaler, picture is very good. However, if you want resolutions higher than 1024x768, or custom resolutions, then the Radeon and a soft decoder are clearly superior. At 800x600 it's a close call, though a Radeon does have better colors.
I don't know how about the Vigatec, but a Faroudja 4000 beats my setup by a close margin, due to it's total absence from noise.
I mean exactly that. The Mpact is old news by now. The Radeon is the better choice, and with today's processors you don't need a hardware decoder. BTW, this is all assuming you'll be using the VGA out on these cards (I assume this since you asked about the Vigatec).
Do a search here on the Radeon, and the Mpact.
I've just noticed: You are right around the corner
A forum community dedicated to home theater owners and enthusiasts. Come join the discussion about home audio/video, TVs, projectors, screens, receivers, speakers, projects, DIY’s, product reviews, accessories, classifieds, and more!