Drats.. It was too dark for me to type a message.. *turns on the lights*
1) Basically it's best if you can find your max limit overclocking on the lowest voltage possible, eg, stock voltage.

Well, none of us would mind bumping up the voltage providing it's within safe range and it could really contribute to some extra overclocking.

2) Right now.. You should know what you wanna overclock.. I was taught to overclock in a way that.. overclocks the proc and the ram invidually first only finding a sweet balance between both. Eg, I will find how high my processor can go, next I'll check on my rams, thirdly, I'll find the sweet balance which gives me the best results and stability without sacrificing much timing and clockspeed from both sides, especially when the DDR2 dividers are rather.. funny.

And about increasing voltage when you wanna lower the timings on higher frequencies.. pretty much depends if your rams can take it actually.

If it can, yes you can try to do that and see if it works.
3) Yes. Keep it under 1Ghz. Just keep the bloody HT Link at 3x since the very start of your overclock la. LOL.

Well, that's my overclocking method.. Others might very well have better or different opinions.