6870 crossfire - 680 gtx

what resolution and how many monitors are you playing on? because for single monitor 1080p resolution, you won't get a noticeable effect from upgrading from two 6870s

1 1920x1200 res monitor,

not too familiar with that size, but considering it's not THAT much bigger than a 1920x1080, I still don't think you'll see a large improvement in preformance... if you can find a gtx 580 for a good price, it should do everything you need and solve your micro-stuttering problem. of course, if you have the cash, by all means go for the 680 if you can find one  

yup either get a 680 GTX if u can get it. or a 7970 for $400-450 (price drop will be soonish)

they will remove your Micro stutter which is cause by X-Fire mid to low end graphics cards.

you will not see major increase in performance however, just the removal of microstutter.

Will there be any increase at all in performance? From some benchmarks the 680 competes with the 590 and 570 sli which are much faster then my 6870 CFX

It will be, around 15-20% I suppose. And of course, 100% increase on games that don't support crossfire.

+1^

but also remember everything's going to depend on your monitor's refresh-rate and your own perception... with a 60hz refresh rate (fairly standard), any fps gain above 60 are essentially meaningless except for benchmarking purposes.

but yes, the micro-stutter will at least go away!

I approve this.