What's your CPU and PSU? Your current CPU might end up bottlenecking a new card and your PSU may not be powerful enough to support it. That's why you should always mention them when asking for suggestions for a GPU purchase.
You might as well get a second 6850 for Crossfire since it will be cheaper and provide you as much performance as a GTX 560 ti, but I'm not sure how well Borderlands 2 scales with it (or if it does at all).
The Radeon 6850 crossfire actually got a honorable mention in Tom's "Best Graphic Cards for the Money" list of this month. So far the only con to this combo is micro-stuttering.
Well i try to edit my post with the following info *How to Ask for Advice about Choosing a New GPU* but its says that im not allow to edit my post
I have a i5-2500k, my PSU is Gold Coolermaster 1000W
And about the Crossfire i'm not interested in this type of technologie because of the power needs (i'm kind of regarding on my "ecological footprint" so i try to use ssd, green hdd, power eficient fan's etc etc etc so having 2x graphic card = 2x power needs and i prefer to go for a new card
Tom ranked it above the GTX 560 ti in his list, and you might like it for not consuming as much power as it's competitors (Max TDP is 130 W compared to the 170 W of the GTX 560 ti) and it's performance should be close to the GTX 570.
Get this card you already have a 6850 a 560 ti wouldn't be a good upgrade it would be better but not enough to buy a new card from your current one not imo anyway and you said your sick about amd so i recommend this http://pcpartpicker.com/mr/dabs/msi-video-card-n660tipe... While I would recommend this without hesitation for 1080p and 1900x1200 and lower, if you go much higher resolutions you really want a full GTX 670, or consider ATIs 7950/7970 offerings. Be realistic in your expectations, but at 1080 and 1200 as well as lower , this card really rocks straight beast.My reasons for my recommendation Always quality and performance and service, if needed.
You said your sick about AMD
Overclocked out of the box
Large performance increase over the last generation
Extremely quiet in both idle and load
Support for CUDA and PhysX
On your monitor res you will be able to play all games and max them out to without even needing to oc.