There is no suitable voltage for anything at all actually. It all depends on your own setup and you have to experiment it for yourself. Each of our setups are different and unique, and overclocking yields different results for all of us. Given the same clockspeed and ram mhz/timing, it does not nescessary give the same result.
IMHO, it is safe to pump as much vcore as you want, but just keep 1 thing in mind, your temps. As long as it is within 55-65 on full load, it is perfectly safe. Procs these days aren't easy to toast. Air cooling, or water cooling, just keep an eye on the temps, it doesnt matter. If your temps are below 60-65 on air cooling on 1.6/1.7v, then it is okay to proceed.
Anyways... i think i hit my walls on my board.. this is as far as i can go and i dont think i can go any further with my current board.
24.367 SPI 1M
Its really pissing me off that i know the fact that it can be pushed further.. my temps on 1.55v max out only @ 50c full load... grrr...
my board is really holding back my rams and the F3