As the subject says, I've got a 3 way crossfire with one 4870x2 and one 4870, not OC'd. I'd like to upgrade to a more modern card with DX11 etc, and maybe something quieter, without breaking the bank and having at least some sort of performance gain and power savings.
Would two unlocked 6950's with the clock speeds and shaders of the 6970 in crossfire be a worthy competitor? I can nab two for around $450. or.. for $100 more, I could get a 7970, but wouldnt that be taking a step backwards from both setups?
Two 6970's should outperform both your current setup and a 7970 by a substantial margin (the wild card is how large of an OC you can hold down on a 7970). 6970's are fairly inexpensive right now- can be had for as low as $300 a piece after MIR. Note: 6950 unlock to 6970 is not very reliable these days and as I understand it many cards can't do it.
The other option is a pair of 6950's or 560 ti's which can be had for sub $400 after MIR if you find a good deal.