6870 CrossfireX + 8120 Bottleneck?

I would say NO. At least not a noticeable one. Would you see an increase of a few FPS going to IVY/SANDY? Yes you would. Would you notice or feel it? Probably not.
I would say you will be very happy with the performance:)

Just throw them graphics cards in and put a slight(or heavy hehe) OC on that 8120 and your ready for some monster gaming!
 

When I Have 1 Card It's At x16 And When I Have 2 Cards It's x8
I've Seen Things Online Where x4 Is Barly Any Less Then x8 Or x16
 

Then you are good.
But, don't believe everything you read about X8 X4 Xfire. I read those articles as well. Then I put 2 X HD 6850s on my X8 X4 board. The results were either no framerate increase over a single HD 6850, or a slight drop in framerate! I tried several games and all had similar results.
 

As long as your overclocked to 4.0 or higher it will be fine.

I am running 2x 6970 and there isn't a game I have that makes the cpu run at 100% while at 4.7 ghz. BF3 ultra maxed settings 1900x1200 I get 90-120 fps with a few dips to high 70s, multiplayer of course.