Pixel filling power
Before we get to the gaming tests, we should set the tone by taking a quick look at the theoretical peak fill rates of these graphics cards, because there are some important differences.

  Core clock
(MHz)		Pixels/
clock		Peak fill rate
(Mpixels/s)		Textures/
clock		 Peak fill rate
(Mtexels/s)		 Memory
clock (MHz)		 Memory bus
width (bits)		 Peak memory
bandwidth (GB/s)
GeForce 6800 Ultra425166800166800110025635.2
Radeon X850 XT Platinum Edition540168640168640118025637.8
GeForce 7800 GTX4301668802410320120025638.4

It's not as if the 7800 GTX shouldn't be the fastest card at high resolutions. The GeForce 7800 GTX has a significant edge over the other cards in terms of multitextured fill rate (and pixel shader power) thanks to its 24 pixel shader units, each of which are capable of laying down one texture per clock. One of the most important limitations of raw pixel-pushing potency, however, is that last column on the right: memory bandwidth. On that front, the differences between the cards are fairly small, and the 7800 GTX barely surpasses the Radeon X850 XT Platinum Edition.

(Although estimating SLI performance is an inexact science, you can more or less just double the numbers for the GeForce cards here in order to estimate the theoretical peak numbers for an SLI rig composed of two of them.)

We don't see a noticeable drop-off at 204x1536 from any of the cards in 3DMark's simple test of fill rate. This test isn't likely to benefit from features like Z culling, so that's not terribly surprising. Real games, however, are a different story.