if it is already factory overclocked i wouldn't really fiddle with the volts; already been tweaked. i goofed around and upped my core from 981 to 1066 but would crash from time to time. i settled down to 1033 with about the same fps as 1066 and things are running smooth.
now remember. those are the clocks on my 550ti not your 560 . . .
I have my Asus GTX 560 DCII OC OC'd to a modest 950/1900/2100 @ 1.062V from a stock 850/1700/2100 @ 1.012V, so you can see that voltage bumps are very small with GPU's (50 mV in this case). Of course there's always risk involved, but most OC programs won't let you go too far with the voltage without some modification, so you should be fine.
The cooling for my particular card is so good though, that I could go much further, if I wanted/needed to. I'm still in the upper 60's in Furmark with my current OC. I have an aggressive fan profile set, but still...
Also, don't even worry about OC'ing the VRAM because there's there's very little performance gain in it. Almost all of the performance gain will be from a Core/Shader OC.