The only thing you’ll lose is bitstreaming HD audio formats. Since you are going HDMI – TV – SPDIF – AVR doesn’t sound like that matters to you.
So you shouldn’t lose anything at all. When the the G / GT series card were first released they did have some colorspace issues but they have been resolved.
The GTX 470 is definitely overkill for HTPC but for gaming and Flash based content it will perform better.
