Upgrade from 6600GT to Radeon 6670

The main reasons I am upgrading is to be able to output HD audio through one HDMI cable, instead of using a DVI-HDMI cable plus an optical cable over SPDIF. I output my HTPC to an Onkyo S-3400.

Sometimes when I am using the computer, change the Onkyo input to Cable to watch TV for a while, and then switch BACK to the computer, the video is messed up and I have to switch the receiver on and off a couple times and then it will come back. I am also hoping the upgrade may help solve this problem.

What other benefits will I see upgrading from the 6600Gt to the 6670??

Thanks!