Depends on the application/game you are running and the temperatures and the PSU
Try increasing +15mhz of core, +100mhz of Memory at once. And keep an eye on the temperature. See if there is any visible flickering or artifacts while running the game. If everything seems fine... then increase it further. If not, reduce the frequencies. In any case.. don't clock memory more than 650Mhz, and 1800Mhz of Memory clock.