Your problem is underscanning. Your ATI card, when running with HDMI, defaults to underscanning the display for some wierd reason, which gives it those black bars around the image.
You can fix this by going into Catalyst Control Center and in "Desktops & Displays" right click the picture of the TV in the bottom left corner (not the big rectangle in the middle, took me ages to figure this out) and click "configure." In here there should be a section where you can adjust the borders by moving a slider to the right.
That should fix your problem.