Sapphire 6850oc CF vs 6850

[mikolv2]

Hello, I have 6850 overclocked to 900mhz but I'm not quite happy with the performance so i wanted a little upgrade. I was looking for buying another one of them 6850's and going crossfire or selling it and buying 6950 or 6970 if i find a good deal.
My question is what do you think guys? What should i get or maybe i should go with nvidia? 570ti?

My specs:
AMD FX 6100 @4GHz
16GB RAM 1600mhz
ASRock 870 extreme3 R2.0
CIT 750watt PSU
and Corsair A70 Cooler

 

[xtreme5]

adding another card for cf is good idea but one gtx 570 is more better idea.