Call of Duty 4: Modern Warfare
We tested Call of Duty 4 by recording a custom demo of a multiplayer gaming session and playing it back using the game's timedemo capability. Since these are high-end graphics configs we're testing, we enabled 4X antialiasing and 16X anisotropic filtering and turned up the game's texture and image quality settings to their limits.

We've chosen to test at 1680x1050, 1920x1200, and 2560x1600—resolutions of roughly two, three, and four megapixels—to see how performance scales. I've also tested at 1280x1024 with the lower-end graphics cards, since some of them struggled to deliver completely fluid rate rates at 1680x1050.

Apologies for the mess that is my GPU scaling line graph. All I can say is that I tried. You can see some of the trends pretty clearly, even with the mess.

The GeForce 9800 GTX tends to perform just a little bit better than the 8800 GTS 512, as one would expect give its marginally higher clock speeds. The result isn't a big improvement, but it is sufficient to put the 9800 GTX in league with the GeForce 8800 Ultra, the fastest single-GPU card here.

AMD's fastest card, the Radeon HD 3870 X2, is a dual-GPU job, and it's quicker than the 9800 GTX. Costs more, too, so this is no upset.

Look to the results at 2560x1600 resolution to see where the multi-GPU configs really start to distinguish themselves. Here, two GeForce 9800 GTX cards prove to be faster than three Radeon HD 3870 GPUs and nearly as fast as four. However, the quad SLI rig gets upstaged by the three-way GeForce 8800 Ultra rig, whose superior memory bandwidth and memory size combine to give it the overall lead.

The frame rates we're seeing here also give us a sense of proportion. Realistically, with average frame rates in the fifties, a single GeForce 9800 GTX will run CoD4 quite well at 1920x1200 with most image quality enhancements, like antialiasing and aniso filtering, enabled. You really only need multi-GPU action if you're running at four-megapixel resolutions like 2560x1600, and even then, two GeForces should be plenty sufficient. The exception may be the GeForce 8800 GT and 9600 GT cards in SLI, whose performance tanks at 2560x1600. I believe they're running out of video memory here, and newer drivers may fix that problem.