What is the max core voltage i should go up to on a amd955 xII

Well AMD says 1.4 so I wouldnt go much over that at all.

I saw someone earlier today that had their PhII X2 550 at 1.6V, which is pretty insane but he hit 4.0 that way.
It depends on if you're discussing a one shot or 24-7 operation. I'd say for 24/7 max I would do is 1.45V. For a one shot I've seen the folks here at tom's go as high as 1.55V to achieve 4.0.

But its your cpu, so you'll have to decide what you feel comfortable with. Also, I think it would be heatsink dependent. Gotta keep those thermals under control. If temps look good at 1.45V, maybe it would be ok.

I haven't had my X4 940 above 1.45V.

Having a good heatsink will keep the temps down that come with a high Vcore, but nothing will stop the electron migration that happens. The higher the Vcore, the faster it will happen. Good cooler or not.

ok who cares what amd says, u can have as much voltage as u want as long as it can keep cool voltage dosnt kill cpus heat does, all the voltage does is lowers life span and the avg life span of a cpu in safe voltage range is 12years if u increase the voltage past that u prob will only get 5years but who has a cpu for 5 years anyways?

^---- Wrong.

Electron Migration is the reality, not just a risk, of overvolting, as alread mentioned by 4745454b . Heat has nothing to do with it.