Step 1: CrossFire With Two GPUs (Vs. SLI)
We can't cover AMD's CrossFire technology without also putting Nvidia's SLI to the test, too.
Unfortunately, we have to limit our detailed assessment of mid-range GPUs to AMD, since we didn't have access to a dual-GPU card like EVGA's GTX 460 2Win in our German lab. Thus, we're conducting a direct comparison between Radeon HD 6870s in CrossFire to GeForce GTX 560s in SLI. This works out well, since both boards are, on their own, pretty evenly matched.
At first glance, it seems that the Nvidia cards don't scale as well in SLI. However, micro-stuttering is less pronounced as well compared to the CrossFire-based rig. It's not gone, though; even with SLI, micro-stuttering is observable.
It is noticeable that a single GeForce GTX 560 (no Ti suffix) is subject to more pronounced frame rate variations, even while the frame rate itself is almost identical.
In this particular title, SLI doesn't scale as well as CrossFire, resulting in a lower average frame rate than two Radeon HD 6870s in CrossFire. The Radeons, however, suffer from visible micro-stuttering, while the slower GeForces subjectively seem to be faster due to the reduction of the phenomenon.
We've run a number of other benchmarks and need to add, for the sake of fairness, that Nvidia's advantage is only evident if its driver is optimized for smooth frame rates rather than raw performance. The company takes certain apps, like synthetic benchmarks and commonly-tested games, and tweaks them to yield higher numbers at the expense of consistency.
Nvidia gets an advantage here, though its cards are slightly more expensive. SLI isn't scaling as well as CrossFire in Call of Juarez, but it does deliver a more balanced sequence of frames.