Personally I do not see the 6970 being an upgrade to the 5870 its just marginally better.
If you were building a new system the 6970 would be the way to go for heat & power consumption reasons.
As for adding another 5870 for CF. Thats subjective. You cant expect 2x the performance as it may not work well or at all with some games, however there are cases where it does almost double the performance. You just have to ask yourself is it worth it? Do you need the performance boost? Is your PC up to running 2 of the GPUs? (cooling, power demand, and also does your mobo support 2x 16x PCI-e at the same time in CF?)
For me it was not worth the 2nd card. I had dual 4850's and gladly "upgraded" to a single 5870, performance was about the same but just having one card was so nice, less bugs/issues, less power and less heat in my room.
I am holding out for what ever card comes next that truly is the successor to the 5870 as a single card powerhouse, also will make sure it has 2GB vRAM.
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