Re: Radeon HD 6850
On my system (5870 normal + 5870 x6, 3 Dell ST2210 monitors) I have issues connecting monitors with HDMI as well. I have 3 identical monitors (Dell ST2210 1920x1080 21.5", DVI/HDMI/VGA inputs) so it isn't a monitor issue. When using DVI they are fine, but I switched the middle one out with HDMI for audio. The picture on it then had a 1" black bar around all sides only on the HDMI monitor. In Catalyst Control Center there is an Overscan adjustment and for some reason they automatically set it too low. Dragging the slider all the way up fixes the issue and makes the picture crisp and clear on my monitor. This may be your problem.
HOWEVER!!!!- if your monitor is a 720p (or even 1080p) television (whether big or small) it may have overscan built in. For some idiotic reason, TV manufacturers think that a pure digital signal isn't clean enough to display normally so they distort it (they think this is doing you a favor, but it is painful and obnoxious). It is mainly a carry-over from analog-signal days when a small amount of overscan would eliminate edge glitches and noise, but on a digital signal the edges don't have noise (if they do, it's the fault of the cable box or other source equipment). Anyways, this will give a particularly nasty picture when using HDMI to connect a PC to a TV with overscan behavior. You can try to tune it out with the overscan slider but usually it is not fixable.
On one 1080p TV I've used, using the HDMI port with a purely DVI (HDMI without audio) signal will disable overscan and give a clean picture, though enabling audio through the HDMI stream will also turn on overscan and kill the picture quality. On my 720p plasma, it has overscan no matter what.
If your TV suffers from the overscan problem and has a VGA port, use the VGA port instead. While VGA signals aren't as good quality, TV's generally do not overscan them and the overall picture looks better. I have my plasma (720p Toshiba 42hp66) connected this way and the picture is fine. Make sure you get the right resolution (some TV's don't correctly report their resolution).