Hi Shockre - first welcome to OCN!
Noticed you don't have your specs listed in your BIO. It makes it easier if you fill out your system specs when asking a question so we know what your dealing with and can provide the best answer for your specific situation. I'm going to assume your PSU can handle two 7850's.
Any
two mid level cards - current series (7000) vs current series (600) cards will always beat any
one high end current series card. Any two mid level current series cards will meet or slightly exceed one high end current series cards. (General rule of thumb).
So obviously the TWO 7850's will beat ONE 680 or ONE 7970.
Since your already own one 7850 the best option for your price / performance wise up front will be to buy another 7850 and go crossfire right away.
Will there be any crossfire issues? Right now there are some bugs AMD haven't ironed out. I'm sure updates eventually will address the issue but no advice on the when. Current 7000 series owners have been dealing with it and working around it best they can.
Crossfire or SLI always come with higher probability for gaming issues or games not taking full advantage of dual card set ups. It may come with temp issues but a well ventilated rig has little issues. If your tower has bad ventilation expect temp issues that will limit your GPU over clocks as well as raise ambient temps in your case.
Non-reference cards run cooler on the GPU because the cards blow the heat off the card right away but dump the hot air into the rig that will limit CPU over clocking head room.
Reference cards run slightly higher Temps on the GPU but losing out the hot air exhaust out the rear will keep the ambient temps down.
Personally I tried crossfire temporarily and ended up selling them for a single powerful GPU solution instead. I had micro-stutter with my 6870'. I moved to 3D Vision with Nvidia anyway so it was short lived.
On a side note
to those perhaps thinking along the same lines that haven't already upgraded to a current series card.
Buying two mid-level performing cards will close the door for upgrade but will give you the most performance up front now.
On the flip side: Buying one high end 680 or 7970 now will give your more than enough performance in any games right now. Single monitor easily or multiple monitor set up just fine. You will leave the door open down the road for when gaming does catch up with you eventually, add another high end card and then go right over the top. At that point two high end cards will easily beat two mid-level cards.
To each his own. As for the OP who already has one 7850 the best smartest move is to go with another 7850 in my opinion.
Good luck bud. Welcome to OCN.
