For high def, I would have to say the 4200 over the 7900. As the 4200 is HDCP compliant, AND passes sound via the HDMI port on the board (5.1 or 7.1 I can't remember) and the HD Decode is about the same regardless (as it's based on the CPU and RAM). If you have the Gigabyte 785G board it also has 128m of sideboard mem used soley for the onboard GFX chip (the MSI does not).
The HD decode need not necesarily based on teh CPU.The ATI 4200 supports hardware based decoding for the HD content . All it requires is the proper SW . I have ATI HD 3200 (780G based ) and able to play 1080p using the hardware ( - Software used WMP Classic - HC .). THe CPU utilization during the 1080p playback is ~5% .