Solving for X
As you may know, the RV670 GPU that powers the Radeon HD 3800 series of graphics cards is a pretty solid mid-range graphics processor, but it's no match for Nvidia's current higher-end GPUs. The contest is close enough, though, that stacking up three or four RV670s makes for a very potent graphics solution. Here's how, in theory, our three- and four-way CrossFire X setups compare to most of today's single-GPU setups—and with two- and three-way SLI setups involving Nvidia's fastest card, the GeForce 8800 Ultra.

Of course, simply adding together the peak theoretical capabilities of multiple GPUs, as we've done here, doesn't account for any of the multi-GPU performance scaling issues we're discussed. But it does give us a sense of where things stand. On this basis, our four-way CrossFire X rig leads all contenders in terms of pixel fill rate and shader arithmetic capacity. Staggeringly, the four-GPU config peaks at over 2.1 teraflops of shader power. Not bad for under a grand! These shader arithmetic numbers are all the more impressive because there's an argument to be made that the GeForce FLOPS numbers you see above may be inflated by a third, depending on how the GPU is being used.

Overall, the three-way CrossFire X solution matches up well against two GeForce 8800 Ultras in SLI and against (if you do the math) a pair of GeForce 8800 GT cards in SLI, as well.

We can test these theoretical capacities with some precision using synthetic benchmarks. These aren't a measure of real-world performance, but they do test something close to the actual peak throughput the hardware can achieve.

Things work out about as expected in terms of finishing order for pixel and texel fill rates. We know from history that this pixel fill rate test tends to be limited more by memory bandwidth than by raw GPU pixel output capacity, but the four-way CrossFire X setup manages to outdo the three-way SLI system despite having less peak memory bandwidth. In the multitextured fill rate test, the GPUs reach closer to their theoretical peaks, which is good news for CrossFire X. One surprise of sorts, if you weren't watching for it, is the multitexturing performance of dual GeForce 8800 GTs in SLI. The G92 GPU has incredible texture filtering prowess with the most commonly used texture formats, although it's only half as fast with FP16 textures.

CrossFire X largely dominates 3DMark's simple pixel and vertex shader tests. Let's see whether it can do the same in real games.