Well I cant tell you exactly....but Skymtl's GB oc'ed 570 will give you a good idea.
Gigabyte GeForce GTX 570 Super Overclock Review
That review shows std 570 performance vs a 570 running at 845mhz. Rember though that GB is binning their 570s to get 845mhz chips. So while you may get one from MSI or EVGA that does that easily...you also may not...or you may get one that does better than it. In either case its about a 2fps on the all important MINIMUM frame rate boost by OC'ing to 845 with a 3 -5 fps increase on the avg over stock 570 numbers. Not exactly a huge dif that is really worth worrying over.