It's different for every card, but most cards are able to achieve somewhere around 850-900 core without any voltage increase, or maybe just a slight bump (mine has a little bump to be stable at 900) and the memory can maybe hit 1150-1200 if you're lucky, give or take a few Mhz.
Don't start with those numbers though. Start with your stock speeds and work your way up slowly while testing for stability and checking on temps in-between. Start with the core, and if you reach instability with your core back it off to a stable speed or add voltage. Once you're happy with that or have reached your limits, then work on raising the memory, once again, slowly and testing in-between. Once you hit instability with the memory that's it, you can't go any further, just back up to where it was last stable and that's as high as it will go.
A couple of notes: When you're gaming, you may encounter what seems like random instability, as was the case with my overclock. It would work fine for weeks, but then give me trouble right after starting up a game before it would even get warmed up. So, if you hit instability after your initial overclocking, you have to play with it little by little until you get it straight. Back off on the clocks or add voltage and go back to playing until you eventually never see instability again.
