The 290x tended to need some help to keep its speed. Did you do this work?
Read http://www.tomshardware.com/reviews/radeon-r9-290x-thermal-paste-efficiency,3678.html
Also, what are you running that needs more than a 290x today? Are you attempting 4K gaming?
So I have a 290x 4gb card atm, and my question is what if any difference in performance would I see between crossfire of 2 290x's vs a 290x and a 390x? I understand that I will only be able to use 4gb from the 390x. I haven't really been able to find benchmarks of 290xCF vs 290x and 390x maybe my google skills are weak, any foresight on this would be awesome

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