I would read some reviews before you buy either. BF3 is currently unplayable on a 6990, and you have people telling you Crossfired 6990's are the ultimate. Tom's Hardware was unable to recommend the 6990 due to the extreme noise levels.
Hd 6990s for the Ultimate gaming experience in Eyefinity setup not a single 1080P monitor. BF3 has some issues with both CF/SLI setups so one game failed to run effectively that doesn't mean that the other games would fail also.
A pair of 6970's won't be far behind and will have their own host of issues to deal with. Think this one through and don't just focus on which setup offers the highest Frames Per Second. At some point, you will want to take into consideration ease of use, game compatibility, aesthetics (noise levels), and the overall ownership experience.
A pair of HD 6970 is ideal for gaming @ a single monitor, i can't play Metro 2033 @ High settings with HD 6970s in Eyefinity setups.
If you are truly looking for the "ultimate" gaming experience, then a pair of GTX 580's w/3gb in SLI is truly the best. Anything above that will be bottlenecked by even the best of the best systems.
HD 6990s in CF, GTX 590s In SLI were tested on a i7 965X, i7 2600K is faster than that, the article didn't state any bottleneck issues and the CF scaling was superb. Saw some benches to 4-way GTX 580s in SLI with 990X @ 5.0 GHz on nVidia 2D surround setup, Metro 2033 achieved 44 FPS or something around that.