what are the rest of your system specs? If you have a slow cpu it isnt going to matter what video card you put in you might be cpu bottlenecked. But yes, if the rest of your system is up to spec, the 560ti would be a noticeable upgrade from a 6850. It also draws more power and you may need a PSU upgrade.
I have a biostar A870U3 which has a x16 and x4 I would also have to upgrade my powersupply and mobo. I have enough for a 570 ti do you think its worth it over the 560ti and will I see a big difference in performance from the 6850?