Testbed:
Motherboard: EVGA 4xSLI
CPU: Core i7 965 XE (Intel EIST andVdropenabled)
Memory: 6GB CorsairDominator 12800 7-7-7-24
Harddisk: OCZ Vertex 2 100 GB
Power Supply: Sapphire Pure 1250W
Case: CoolerMaster HAF X
Fan Controler: Kaze Master Pro 5.25"
Operating System: Win7 64-bit
Nvidia 270.51_desktop_win7_winvista_64bit
AMD 11.4 Preview
So, our multi-GPU system is made of three Cayman GPUs. The HD 6970’s GPU runs at 880MHz while the HD 6990’s GPUs run at 830MHz. 
Note that when using two different graphics cards in CrossFireX, the faster card will downclock to the level of the slower card. However, we used the bios switch on the HD 6990 and boosted its GPU to 880MHz, in order to make it identical to the one on HD 6970. 
The scores show that slotting the HD 6970 next to a HD 6990 may improve performance up to almost 50%. We did not forget the HD 6990 quad CrossFireX, we simply did not have two of them.
Noise and Thermals
Naturally, the HD 6990’s temperatures are higher than those on the HD 6970, but the thermals end up just fine. We were more worried about noise, because the cards can get pretty loud in 3D mode.
Afterburner tool detected the fan seriously speeding up from time to time. We’re sorry to say this, but the fan noise barely allowed us to enjoy our gaming sessions.
Power Consumption
Consumption is unwise to ignore when dealing with CrossFireX and SLI setups, even if you don’t care much about your power bills. For instance, you should make sure that your TriFire rig with a HD 6990 and HD 6970 boasts a 1000W PSU.
The HD 6990 card consumes around 37W in idle mode, which isn't half bad considering that a single HD 6970 consumes 22W. On the other hand, HD 6990 consumes around 335W and if you intend to overclock it, you should know that it can draw up to 370W. This means that it will draw about 125W more than HD 6970, via two 8-pin connectors.
Our test rig drew about 200W more with Quad GTX 590 than with the TriFire (HD 6990 + HD 6970). During intensive operation however, our rig with TriFire inside consumed about 845W.
Conclusion
In short, if you have an HD 6990 and you crave even better performance, then adding an HD 6970 may save you quite some cash and allow for gaming at maximum resolutions.
