Both the 6970 and 580 are single GPU's. I'm unsure if you had that correctly by a couple of comments.
To be more help, you may want to mention what your monitor resolution is, and your CPU/RAM/PSU setup.
As far as the pro's and con's of dual vs single setups:
+ Very fast, the 6970 in crossfire (with two of them) is considerably faster than the
+ Cheaper for the performance at the higher end, generally.
- Not all games support CF/SLI, but most will.
- Some games don't run well with CF/SLI and cause glitches (i.e. stuttering). Most will be fine.
- cuts down on air circulation and runs hotter.
- requires more power (bigger PSU)
Unless you are using a very high resolution or multiple display setup, I recommend a single card.
Are you talking about the 6990 and/or the GTX 590? Those are the dual GPU cards.
-Able to add another to get Quad SLI/Crossfire
-Have lots of life in them
-When they get outdated its more expensive to replace...
-You can SLI/Crossfire two or more cards and have it equal or rival the dual GPU
-You can only pair two of them together
-Power Hungry (But most SLI/Crossfire setups are)
-2x the Chance to get a DOA (Two GPU's in there)
-Drivers can be Iffy (Crashes/BSOD's/Stuttering/Ect)...
-Games will see it as a SLI/Crossfire setup, and not all games are able to use SLI/Crossfire
I would never buy a dual GPU. If I had that much money to spend on a PC , I would get two or more GTX 570's or ATI 6850's 2GB. Plus unless you are running over 1080p, you will not need over a 6950 2GB dual Crossfire setup (And you would only need that if you play graphic intensive games such as Crysis/Metro 2033).
Another thing to consider when SLI/Crossfire is that it does NOT give a 200% increase like you would think. Scaling has gotten better and better, but it still doesn't get 200%, even on the dual GPU cards.
Another thing to think about, when you buy the dual GPU cards or SLI/Crossfire, you need to think about what CPU you have, how much RAM you have, and Resolution you are running.
Dual gpu cards tend to have much shorter life spans than that of single gpu cards and there are numerous problems that are not even listed in this thread thus far. There are occasional compatibility issues with older boards or boards that are modern but do not have the micro code in the bios needed to boot the switch on such cards. With out the micro code the bios will not be able to boot the card due to the switch. The switch is it's own device that is normally invisible to the end user but present to bios utilities. Failures have been observed of the switch in some cards that left one of the gpus unable to function at all or corssfire/sli no longer functions but both gpus are still resent. Power is one issue while cooling in well known.
GPUs are the central cores on a video card. In the last few years there's only been 3 released, the 5970, 6990, and GTX 590. They have two gpu cores on a single PCB (single card).
The 580 and 6970 are both single gpu cards. The 1.5 and 2 represent the amount of video ram (VRAM). The VRAM has very little to do with being dual or single gpu cards.... sort of. The 590 has 1.5gb per core, so it has 3gb VRAM and similarily the 6990 has 2gb per core, so 4gb. It's important to know that a dual GPU card acts the same as SLI or Crossfire between two discreet cards. This means that the VRAM is not additive - so the 590 is still only able to use 1.5gb of VRAM, and 2gb for the 6990.
There isn't much point in buying a dual GPU card, because you can SLI or CF discreet cards for less money and more performance. The only point is to do quad SLI/CF (ie: 4 gpus) on a motherboard that supports 16/16 PCIe lane bandwidth - compared to 8/8/8/8 for 4 discreet cards.
But there's problems with the newest ones being very very hot and very very power hungry. If you just buy two cheaper cards (in this example, 6990 vs CF 6970) you'd be able to overclock the 6970s a lot more because each card is making half as much heat and has to handle half as much power.
Some lukewarm posts just like going to an apostate watered down church :s
Just basic info but then again people don't bother to live and breath the subject for a while. How about dual planar cards or quad planar now that is when things begin to get more complicated as they say. Pass through cables, bridges, switches, power, cooling, drivers, performance scaling ect. Modern dx9 era gpus on up have a limit up to 4 gpus in the same rendering for the same application such as sli or crossfire. How about different rendering models such as AFR or SFR ect how about the differences in these modes between ATI and Nvidia. Tile rendering, dynamic SFR, 3 or 4 frame AFR modes and much more.