As opposed to the midrange graphics cards like Radeon HD 6870 and HD 6850, the top-end single-chip Radeon HD 6970 does not often double the performance when used in a CrossFireX tandem. There seem to be several reasons for that. One is that the speed of a high-performance graphics subsystem depends more on the overall system performance. Then, the Radeon HD 6970 itself is rather a new product, so its driver may not have been optimized properly as yet. Besides, the CrossFireX technology is not supported well by the developers of certain games. But for all these problems, a Radeon HD 6970 tandem can indeed be twice as fast as a single HD 6970, particularly in such games as Crysis, BattleForge, Aliens vs. Predator (2010) and Sid Meier's Civilization IV. We also observed a 100% performance increase in the Unigine Heaven benchmark. The scalability varied from 80 to 90% in some more games as well, which is a very rewarding result. Considering this, we suspect that the potential of CrossFireX with top-end graphics cards is just not yet revealed in its fullness. We hope this technology will be optimized further to deliver a stable and consistent increase in performance across all games (especially in terms of the bottom speed).
Comparing CrossFireX and SLI with respect to top-end graphics cards, we cannot prefer either of them because their standings depend much on a particular game, and we don't think that users will buy a pair of such products for a total of $700-750 to play only a few select games. People who use such solutions naturally expect high performance in every game they launch. As for other aspects, our GeForce GTX 570 SLI tandem produced less noise, had lower GPU temperatures and also came at a lower cost than the Radeon HD 6970 CrossFireX, but you must be aware that replacing the cards' native coolers with something more efficient is not just desirable but vitally important for your multi-GPU configuration to have a long service life and be more or less comfortable to use.