Refresh Rate Problem =( plz read

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!