As much as you can get away with. Unless you do something drastic, it's highly unlikely you'll fry your card.
Start off by adding 5mhz to each. Play a game for an hour, add another 5 mhz to each, play a game for an hour, repeat. Once you get to 170/210 (if you have an ultra) or so, you're probably maxing it out, or close. A non-ultra's memory might max out at 160-175, but it's still possible the core could hit 150-170. Might want to take smaller steps and use it for a day when you get to those higher numbers.