Hi, I have a Sony Bravia 46 inch 1080P rear projection TV, I hooked up a computer with a diamond HD 3850 video card and was immediately able to set the resolution to 1080p @ 60hz, which the TV is supposed to support. When I try to hook up a computer with a sapphire HD 3870 and set it to 1080P @ 60hz (I have to force this refresh rate on this card) the image becomes too large for the TV. I have tried multiple drivers.. and no luck. The best setting I have gotten with the 3870 is 1080P @ 50hz or Optimized 1080p @ 60hz (not true 1080p, its 1776x10xx), but some games do not support that resolution so I am forced to use 24hz.. which tears. I simply don't understand why an HD 3850 would be able to produce 1080p @ 60hz and an HD 3870 is making the image to large. They have the same display engines. help please!
I'm having the same problem with a Radeon 9000. Like you, if I set the resolution to 1080p @ 60hz, the image is too large for the physical dimensions of the display (37" Panasonic LCD). Ultimately, I have to downgrade the resolution to 1080i @ 24hz to get the image to fit properly.
The curious thing is that I don't have this problem when I connect this card to my Dell 24" LCD monitor (1920 x 1200 @ 60hz).
I suspect the problem has to do with the native resolution support built into the firmware of the video card. This may explain why your two video cards achieve different results even though they have the same gpu.