If I remember correctly, the memory of two cards in a Crossfire configuration is not cumulative. Instead, you'd have two 6870s in Crossfire with 1GB of graphic card memory (not 2GB). So your comparison should be against a single HD6970 vs two HD6870s in Crossfire. There should be a performance chart around here somewhere with that information.
Generally speaking you are better off going with the more powerful single card rather than two less powerful cards. When it comes to VRAM two 1GB cards effectively only use 1GB or memory, as the same VRAM data is copied to both cards. The 6970 2GB would use the full 2GB, if you can find software that actually needs that much, or if you game across multiple monitors.
While the 6000 series does have very good crossfire scaling, and I think the 2 6870s could theoretically slightly outperform the 6970, a lot of people have been reporting driver issues, leading to microstuttering and reduced framerates in some games, particularly when crossfiring 2 6870s.
Alrighty thanks for the answers guys, think ill go with the single 6970 as i am only planning to use 1 monitor... However, will there be issues with 1920x1080 on a single card or am i just underestimating the abilities the graphics card itself?
It does seem that there are a fair bit of people with the 68xx cards, who have had issues with crossfire not working correctly, although I haven't heard of them having microstuttering, but maybe they have. I have not seen anyone with the 69xx cards having the same issue. I also believe it's generally better to have 1 good card instead of 2 medium cards. There are just more things to go wrong with crossfire/SLI setups.
Ok... guys actually its not exactly true that 2 crossfired GPUs use only their own Vram, its just that the second cards Vram is used very poorly.
It is my sugestion as well to get the 6950 with 2gb. You will not get the performace of 2 6870, but will have a future proof chance to crossfire 2 6950, and thats a beast.
About the Crossfire problems, they do exist, and always will, same as SLI, so your better off with single probably (also im guessing power draw will be significantly lower on 1 gpu).
Time out. 6870's in crossfire perform significantly better than a single 6970. In fact, they perform better than a single GTX 580, which is faster than the 6970. Here is a blurb from Tom's last "Best Graphics for the Money" which specifically discusses this:
With regards to the 2GB vs 1GB, don't get hung up on that. At 1920x1080, you're not going to notice ANY DIFFERENCE at all in performance between the 2GB and 1GB versions. Look up any kind of benchmarks for these cards vs the 1GB version at the same clock speeds. They perform exactly the same. The same is true of 6950 2GB vs 1GB. Until you get into higher resolutions (2560x1600, multi-monitors, etc) the VRAM on these cards isn't limiting performance and will not provide ANY performance benefit.
There is the potential to have driver problems with multi card setups but it is the exception, not the rule. The potential to have setup difficulties shouldn't be a deterrent to making a much more logical choice unless you have specific reasons to go with a single card. If driver problems were at a significant level, you wouldn't see review sites recommending them.