kfc, to overclock the memory you want to change the memory ratio......I am running PC2100 right now....my FSB is at 195 and my memory ratio is 80%, putting it at 156 mhz......
i'm running at 1.7 vcore and i'm stable. I'm gonna set my vcore limit at 1.75, because I don't like high temps, hehe. I thuroughly enjoyed my load temp of 36C when I had my barton 2500 and 8rda+ at stock, heh. I'm running at 42 load currently.....
As to how high is too high for vcore......test. Just keep your load temps below 60. Once you pass that, you might start to see system instabilities. I've run safely with my 1600 pally and 1000 tbird at 65C, but a degree or two higher would cause crashes.
Anyhow, put your vcore as high as your temps will allow. Keep your temps below 60 and you'll be fine.