The 6990 is faster than the 7970. I still own both cards and have done a lot of benchmarking with both and the 6990 runs roughly 10% faster than the 7970 on average depending on the game and on the benchmark you are running. That is comparing stock for stock and overclocked for overclocked. The 6990 is a bit of a power hog requiring 375W to run at stock speeds and if you want to overclock it you will need to be able to provide up to 450W to a single pci-e slot in order to do this. If you have a decent motherboard and power supply then this is no problem. On the other hand, the 7970 only needs 250W to run at stock and I really have no idea what kind of power it requires overclocked since that mostly depends on how high you have it clocked but it will still be a lot less than 450W. The 6990 is a dual gpu which is essentially two 6970's crossfired on the same card. The 7970 is a single gpu. The first problem this creates is if you are trying to play a game with the 6990 that does not support crossfire then you are out of luck since you have no choice but to run crossfire. At any single monitor resolution the 6990 will play just about anything maxed. Once you get into eyefinity resolutions like 5760x1080, a single 6990 is going to have a hard time with some games running maxed out. This means you now need another card to crossfire but the problem is that there are more games that will not scale well with more than 2 gpu's than there are games that will. Games like bf3 and dirt3 will run well with crossfired 6990's. I have found that 4 gpu's running would all be above 85% in these games in the worst case. On the other had, a game like Skyrim only works with up to 2 gpu's. A 6990 running skyrim in ultra at 5760x1080 with FXAA enabled only gets rougly 45 FPS. If you go with 8xAA instead of FXAA the FPS is even lower so you have basically hit a wall with the 6990. Since the 7970 is a single gpu that comes close in performance to the 6990, you can simply crossfire another 7970 for games like Skyrim it is a big advantage. I actually had two 6990's for a couple of months and found they were great in some games but I found myself playing more games that were good for up to 2 gpu's so having 4 gpu's began to be a real pain in the rear end.
yes that is almost like the 680 and the 590... the 590 is only a little better than the 680 but like stated before, not all games works in SLI and somegames can present problems like low performance gain on SLI or microstuttering, also if you wan't to add another Quad Sli is even worst in compatibility than SLI a single card solution is always the way to go.
so i would say, the 6990 is a great card, and if your system have it already, there is not need of replacing it so soon, unless you are thinking in Crossfiring in a close future. The 6990 is a good solution but a single card solution like the 7970 is always the way to go, safest and more future proof solution.
As a general rule, a single GPU solution means fewer hassles than a multi-gpu solution. With multiple GPUs you have to worry about games supporting crossfire/sli and also even when the game supports xfire/sli scaling well, you have to worry about microstutter. Furthermore, the reported noise of the 6990 is enough to warrant the 7970 which a cooler, quieter, less expensive card that packs a larger 3GB frame buffer. I know the 6990 has 4GB of VRAM but it is split in 2GB segments for each GPU...hence effectively the 6990 only uses 2GB of VRAM.
I, personally, found the 6990 to be fairly quiet until you passed 50% with the fan speed which is the same for all AMD reference coolers. They are all loud when you start going past 50%. Even the 7970 gets loud when you ramp up the fan but the difference is the 6990 had no non reference choices and the 7970 has lot's of good non reference coolers.
A month in on this but I can say that if you want to water cool your graphics card(s) for optimal performance then 6990(s) would be your best bet. Also you need a good CPU to avoid bottlenecks so anything like Thuban, i7, i7 extreme editions,or maybe FX-81XX. Also for 1 6990 minimum 850 PSU and for 2 of those you should get 1500 for some headroom.