Yeah but you have to realize what you've dedicated yourself to as well.
By buying 5850's sure you save yourself the money, however you have a lot more liability... if the whole bitcoin market crashes and is wiped out, or people want to jump ship, the market will be flooded with 5xxx series cards as said previously, and that means they will even further decrease in value since everyone will be undercutting eachother as well.
Secondly, you are sacrificing a good option to also game with the setup. The 5xxx series have a lot of crossfire issues with playing some modern games, and especially any older games.
Finally, you leave yourself without the ability to re-sell them for much in the next year even, because in Fall the 7xxx series are going to be out and if you think about it in current times, no one right now wants to buy 4xxx series do they?
You've completely dedicated yourself towards a dedicated rig, with hopes that the market will support you, and you have no re-sale value.
As you can tell I am a much bigger supporter of buying 6970's as I have done so myself. (Technically 6950's unlocked into 6970's)
You may have just sold me on a 6970...I'm due for a hardware upgrade anyway.