Well, first off, if you're not comfortable with editing your registry you should avoid doing it. If you've already done it exactly as that post described, and it still isn't displaying correctly after restarting, I would remove that registry key. But, and I can't stress this enough, make sure you're just removing the key you created. If you're not 100% sure which one you made, then leave it alone.
As far as what's going on, I think it's an underscan setting. Underscan and overscan are compatibility settings dating back to analog television. Things like size and position and actual resolution versus displayed resolution. When older AMD graphics connect via HDMI, they often think they're connected to a TV and turn on underscanning to make sure the whole image is displayed.
Normally you'd just change the setting in your AMD Catalyst Control Panel, CCC for short. Otherwise you can try the registry tweak mentioned above, if you're comfortable with it. Beyond that, you can try connecting with a different type of cable, such as DVI or VGA, although that last is analog and may have lower quality.