What we need to do is verify that the image quality between ATI Radeon and NVIDIA GeForce cards are pretty much the same so we can objectively measure performance.

Now looking closely you'll notice a little color-tone difference. This scene has a lot of fog consistently moving from the right to left, which is the cause of that color tone difference. You can actually see that if you look at the end of the road and then compare with the Radeon shot. Due to resizing the images from 2560x1600 to ~ 600 pixels wide makes this a difficult thing to show.

Anti aliasing at 4xAA
The reality is that there is very little difference to spot with the naked eye. AA however is managed slightly different between ATI and NVIDIA. Let me show you:

Now to the left ATI's AA pattern, to the right NVIDIA's. NVIDIA does has slightly better softer edges. But mind you, this is a crop from a tree/branch at 2560x1600, and then then magnified 300%

High resolution textures and filters
For the second image quality check we take Brother in Arms: Hell's Highway. No AA here, we focus on texture filtering and texture quality.

As you can see the differences between the two brands is astonishingly similar. Color tones can be a little different by the way. BIA has an interactive rendering engine with clouds and lighting.

Look at the more textured objects like the backpack and compare that among the two. No difference. The same for the helmet and the logos on there. This constant quality applies throughout the scene. Again this is a crop taken from a 2560x1600 sized screenshot. So the only difference I could spot is the roof of the shed in the back. I just have a really hard time deciding whether that is a result of texture filtering, or just a distortion.