I wanted to give you a better answer than just "it'll be faster" so I've been hunting for Fable 3 and Witcher 2 benchmarks. Unfortunately the pickings are slim.This german article
benchmarking Witcher 2, whilst obviously in German, has a 6870 managing an average FPS of 30fps @ 1920 x 1080 with an older i7 860 (socket 1156) model.
Perhaps try and recreate their benchmark scenario and see what your system manages?
If you are going to do the upgrade, an i5 2500k is an excellent choice. Well I say upgrade it's more like a new PC!