Call of Juarez
ATI’s Catalyst seems to manage memory more effectively than Nvidia’s ForceWare: the ATI Radeon HD 3870 is only slower than the GeForce 8800 GT 512MB at 1280x1024 and then goes ahead, being only second to the Nvidia GeForce 8800 GTX. This impressive result has little practical worth, unfortunately, due to very low performance. The GeForce 8800 GTX cannot even provide 25fps at 1280x1024 if you use the highest graphics quality settings together with FSAA.
Call of Duty 4: Modern Warfare
Alas, application-specific driver optimizations are still a weak point of graphics cards from the former ATI Technologies. Although the ATI Radeon HD 3870 is no worse than its predecessor, it is not enough to provide a comfortable frame rate even at 1280x1024. You can try to play at such settings, but you are not safeguarded against slowdowns below 23-24fps. You can increase the speed of the game on an ATI Radeon HD 3870 by disabling FSAA, but this won’t do for a Radeon HD 3850.