There are a number of improvements in the video and audio departments that will sure please the HTPC crowd. For starters, AMD has upgraded the 780G's HDMI 1.2 support to version 1.3 and jumped from UVD 1.0 to UVD 2.0 on the 785G. The ability to output multi-channel LPCM audio output over HDMI has been added, as well, although TrueHD and DTS-HD audio remain unsupported.
Aside from that, the new chipset brings full AM3 and DDR3 support for rather less than the relatively pricy 790 boards you'll need at the moment. For a more detailed look at AMD's integrated 785G graphics platform, you can read some reviews at: PC Perspective, Tomís Hardware and The Tech Report.