All on ultra with ubersampling off, and overclok you CPU you can get up to 4.5ghz on air.

Overclock a Core i5-2500K to 4.5GHz
1. Enter the BIOS by pressing Delete as the PC starts
2. Enter the Advanced menu by clicking the button in the top-right corner
3. Set the CPU multiplier to 45x
4. Enable Load-Line Calibration
5. Set the CPU voltage to 1.3-1.35V
6. Set the CPU PLL to 1.9V
7. Set the VCCSA to 1.1V
8. Set the VCCIO to 1.106V
9. Hit F10 to save and restart
http://www.bit-tech.net/hardware/cpus/2011/01/03/intel-sandy-bridge-review/1

How much difference will the overclocking make?

You don't need to do any of that. You could totally ruin your computer if you don't know exactly what you are doing. Besides, your system is plenty powerful enough to run the game on high settings. And it looks gorgeous.

I have,
AMD Athlon II 630 quad core at 2.8
4 Gb mem.
XFX 5770 1 Gig mem

Your gear is better than mine and I run on high settings at 45 to 60 FPS.
I have V-sync on.

You really don't want to do that. If you want to overclock you need to do it gradually. There is no mention here of stability testing.
DO NOT enter somebody else's settings as everybody has different case and CPU cooling options. If interested look at some forums and you will learn it's a slow process that yields the best results.

search specific info about safe overclocking on your cpu so you can do it without damaging your system.

My i5 750 2.6 is OCed to 3.2 which i think should be enough for the Wticher 2. It's a totally safe OC since i don't even change voltages (which is the main reason for shortening cpu life) for it. In my case the only thing i do is change BLCK up to 160 instead of 133.

Anyway don't OC your CPU if you are using the stock cooler

The OP should be just fine with his rig.

I'm playing on a mix of high and ultra settings (i.e. started with Ultra spec but turned ubersampling off, SSAO off, Cinematic DoF off, 4xAA on, texture memory large) at 1440x900 resolution using:

Athlon 64X2 4400+ OC'd to 2.9GHz w/ liquid cooling
4GB of 1066MHz DDR3 RAM
460GTX 1GB w/ stock cooler

And I'm getting a consistant 30FPS in heavy action scenes, all the way up to 55FPS in smaller interior locations.