Radeon 7000 doesn't currently support Crossfire and only 6800 and below have serious problems with micro-stutter, all of which go away if you have three or four GPUs.
I'll look for proof later, but Tom's did many articles that show this stuff so far and Cleeve himself has said on several occasions that GCN Radeons don't have fully operational Crossfire support in games right now. You may have read the articles here on Toms about micro-stutter and similar problems a few months ago. There was a similar article that featured the Radeon 6870X2 back in early 2011 or late 2010 and it has been in the featured articles recently, it went on about triple Crossfire and SLI scaling too. It proved that three GPUs don't have micro-stutter problems like dual GPUs do and that a fourth GPU is often not very useful for anything.
I don't have the proof right now and I'll have to look for it, but I can explain why this works this way, to an extent. However, once GCN Radeon 7000 Crossfire suport is fixed, I don't know how well it will perform. It is expected to at least match Radeon 6900 (currently the best multi-GPU scaling cards), but that is pure fan speculation.
Think of it this way. Multi GPU tech now has one GPU render one page while another is rendering the next one instead of them working together on the same page. This means that the optimal patern looks like this:
a is the first GPU, b is the second.
However, sometimes we see it look like this:
We have two frames shot out right very quickly, then a larger space between the next two. Dual GPU platforms can do this, this causes micro-stutter. Having three GPUs should look like this:
It is a lot more unlikely for three GPUs to have a frame ready at the same time, so micro-stuttering is about as good as a single card solution, sometimes even better. However, FPS doesn't scale with three GPUs as well as it does with two. I think it's a fair trade off because if you have a micro-stuttering problem, it doesn't matter how high your FPS is, it still looks like crap. The scaling also helps because it means that instead of one GPU working to get 20 FPS or two woring to get 40 FPS, you can have three working to get 50FPS. Think of this: each GPU has more time to render each frame, so if it runs into a particualrly heavy frame, it has a lower impact on performance and a much lower chance of throwing the frame order out of whack.
three GPUs have a different problem too, but it's not nearly as bad, especially at high FPS. You need to wait an amount of frames equal to the number of GPUs before your mouse can register a change in position. Dual GPUs have almost double the FPS so it's not a big deal. Three GPUs can have a bigger deal if you have such high settings that the three GPUs can't get out a high enough FPS. If you have high FPS, waiting three frames is no big deal. If you have low FPS, well then it increases latency, that can be almost as bad as micro-stuttering.
Two 6950s are better than one 7950 at stock, but the 7950 can be overclocked to roughyl equal two 6950s or maybe even two 6970s. I suggest the 7950 if you like to overclock, the 6950s if not. One 7950 at stock clocks is about as good as a GTX 580, sometimes a little faster.
I asked on a different forums about my GPU and they said:
[quote name='Zpoon' timestamp='1331152055' post='14753746']
The build looks good overall. I'm really happy with ASUS's UEFI BIOS so you should have fun with that. Except..
Holy hell what's with your GPU choice. Any reason why you're going all out and purchasing that type of GPU? A significantly less expensive 6950 will be able to run any current game on high settings no problem, and will provide adequate future proofing for the next few years. Hell you could even crossfire two 6870s and get comparably equal power and still be 150 bucks cheaper than the 7950.
Unless you're doing heavy CAD/workstation work I strongly suggest you do not buy that graphics card. No current, and I see no future game within the next few years using that type of hardware. And future proofling like that doesn't work with the way GPU's are dropping right now.
Two 6870s have horrible micro-stutter problems. Overclocking is easy. Overclocking the 7950 will go far beyond two 6870s even if they were overclocked too (7950 overclocks FAR better).
What video cards you need depend on your monitor's resolution. For 1080p, I recommend a Radeon 6950. For 1920x1200, I recommend a 6970. For 2560x1440 and 2560x1600 and 3D 1080p, I recommend dual 6950s or the 7950, if you overclock then the 7950.
If you only have a 1080p monitor, then you are looking at graphics power greater than you need. The quote you posted isn't very well informed. It is actually not a bad idea to do future proofing and the 7950 is the best option for that right now. Also, there are several ays to make use of even greater hardware than I recommended.
For example, if you wanted a triple 1080p Eyefinity setup, then you might want three Radeon 6970s or a Radeon 6990 and a 6970. However, a 6950 does NOT provide good future proofing. It is the minimum you want to play the most intense games out today fully maxed out at 1080p. It won't be able to play more intensive games released later on at 1080p and maxed out settings unless you overclock it.
In the case of over-clocking, then the 6950 is adequate for future proofing for at least two years (unless something REALLY intensive shows up from out of seemingly nowhere).
6870s would be great if they did triple Crossfire, then I could recommend them more. however, the only way to get triple Crossfire 6870s is with a 6870X2 and a 6870. The problem here is that all 6870X2s only have 1GB of RAM per GPU, not enough for it's GPU performance so it is crap for going beyond 1080p, which is the whole point of a card that fast.