So, what's a dual 5k series GPU I Can get for under 400$ that would be faster than dual 6870s  

Guessing 2 of these?
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125318

I'm looking at the GPU mining comparison charts and it seems like those 2 cards aren't far off from each other.
https://en.bitcoin.it/wiki/Mining_hardware_comparison
Also, I would have to overclock the 5850 up to maybe 900MHZ before hitting 300Mhash/s.....hmm
So....5850 or 6870?