DVI and HDMI output the same signal, thus resulting in the same image quality. The difference between the 2 connection types is that HDMI has "built in security" of sorts and allows audio to pass over the same cable. Simply switching from the DVI->HDMI cable used for your 8800GT to an HDMI->HDMI cable with an HD6850 won't inherently improve image quality. If you see any improvement in image quality it will be strictly due to the graphics processor and related graphics/image settings used in the catalyst control center software.