first make sure your memory is running synchronously with the cpu (1:1 ratio). that means both at 133mhz (266mhz ddr for memory). if your memory is set to 167mhz (333, pc2700) then once you start to increase your fsb, the memory will likely be the first limiting factor since you're pushing it beyond it's rated speed. if you keep it at 266mhz, you will have a good amount of overhead so you know for sure the memory isn't limiting your overclocking.
if you just now corrected that, go and try moving your fsb up a little more (2mhz at a time, booting into windows, benchmakring) and see if you can get higher without a voltage increase.
once you find the limit, increase the cpu voltage by 1 increment (.025v) and try moving the fsb up again until you find it's new limit. and always watch your temps! basically keep doing this until a step up in voltage doesn't get you any farther with your fsb. then you've reached the cpu's actual limit and no amount of voltage is going to help it. at which point, you can look at your temps, stability, and the extra mhz you've gained and ask yourself, "is it worth it?"
very few people run their processors at their utmost limit (for stability and temperature reasons, mostly). don't mess with the ddr voltage since you're not overclocking your memory.
most of all, be careful and take small steps. i don't take any responsibility for burnt'd proc's.