Always take it one step at a time. I'm personally a little lazy when overclocking and like to take big jumps, but still always use multiple jumps not one big one. I would leave voltage alone.
From there I would personally go up by 100mhz and test because I have seen a lot of AMD cards get an extra 100Mhz easy. From there I would jump up by 50Mhz until it fails a graphics test. Then I would jump back down to the last stable position and jump up by 10Mhz until it fails the graphics test again. Then I would increase by 5mhz and test and not go any further.
Do the RAM the same way. This is a quick way to overclock. Not quite as safe, but I am not so worried about damage if I over jump the ability of the card a little and have to back down.