I went from Q6600 myself and saw a great difference, especially once overclocked. There was someone else on another forum comparing the two, here is his results
Old system (min/avg/max fps): E8400 @ 3.8, 4GB DDR2 @ 840Mhz, GTX 460, Win7 64
SC2 - 50/64.5/91
NS2 - 33/49.8/60
Civ5 - 27/50.2/64
Skyrim - 31/48.9/61
New system stock: i5 3570K @ stock, 8GB DDR3 @ 1600 9-10-9-28, GTX 460, Win7 64
SC2 - 68/98.1/130
NS2 - 57/65.5/71
Civ5 - 39/53.4/64
Skyrim - 45/56.9/62
New system OC'd: i5 3570K @ 4.4, 8GB DDR3 @ 1866 9-10-9-28, GTX 460, Win7 64
SC2 - 69/105.6/132
NS2 - 55/72/95
Civ5 - 46/57.7/63
Skyrim - 44/57/62
About the tests:
SC2 - Single player all in mission, average of 2x repeatable run of sending all troops to zerg base. All graphical settings set to Ultra, no vsynch.
NS2 - Natural Selection 2 beta 207, all settings set to max. Mineshaft map, 2x repeatable run w/environment and gun fire.
Civ5 - Max settings w/4x MSAA. Running a few troops from one city to the other, 2x repeatable run. FPS seems capped around 62FPS.
Skyrim - Max settings, 2x MSAA, 8x AF. 2x repeatable run in Eldergleam caverns. FPS seems capped around 62FPS.
Credit - Avalon/Anandtech Forums
Anandtech has a cpu comparison chart
they dont have the i5 3570k yet but you can compare your e8400 with 2500k and 3570k should be just a little bit better than that. Performance increase can be as high as 250% at times.