The basic metrics
I'll admit I've been holding back on you about just how powerful the X2 really is. If you consider it as a single card, it has substantially more theoretical peak throughput, in nearly every major category, than any of its competitors. Here's a look at the basic math.

In terms of pixel fill rate, filtering of FP16-format textures, memory bandwidth, and shader arithmetic, the Radeon HD 3870 X2 leads all other "single" graphics cards on the market today. Most notably, perhaps, it's the first graphics card to pack more than a teraflop of shader power. (And I'm using the more optimistic set of shader arithmetic numbers for the GeForce 8800 cards; another way of counting would cut their numbers by a third.) So long as the X2 can scale well to two GPUs and use the power available to it, it should be the fastest card of the bunch.

I've chosen to test the X2 against a range of different graphics options, as you'll see in our first set of synthetic benchmark results below. Perhaps the most direct competition for the X2 is GeForce 8800 Ultra. Ultras typically cost quite a bit more than the X2's $449 price tag, but board makers offer a number of "overclocked in the box" GeForce 8800 GTX cards like this one with near-Ultra clock speeds. The stock-clocked 8800 GTX, meanwhile, has essentially been rendered obsolete by the GeForce 8800 GTS 512, which offers very similar performance at a much lower price, as we found in our review. As a result, we've allowed a stock-clocked Ultra to be our sole G80-based representative.

The single-textured fill rate test is essentially limited by memory bandwidth, and the only solution that can top the X2 on that count among our group is the Radeon HD 3870 in CrossFire. If you'll recall, the 3870 has a slower GPU clock but a higher memory clock thanks to its use of GDDR4 memory.

When we get to multitextured fill rate, the X2 looks to be limited by the texturing capacity of its GPUs. The cards based on Nvidia's G92 don't reach anything close to their (rather staggering) theoretical peak filtering capacities here, but the GeForce 8800 GTS 512 and the 8800 GT in SLI manage to outdo the X2.

The X2 takes the top spot in three of the four simple shader tests in 3DMark06. I have no idea how the Radeon HD 3870 is able to produce much more than double its vertex throughput in CrossFire than with a single GPU, but the voodoo magic seems to rub off on the X2, as well.

Anyhow, those synthetic tests give us a sense of the lay of the land, but they're by no means the final word. Let's look at game performance.