run prime95 and see if it crashes or blue screens. generally with the PhII Denebs you can get them up to 1.55V (i wouldn't suggest higher) without serious risk. your big concern is heat. my deneb really gets hot over 1.45V, most do. if you can't run prime95 for a few hours you're not stable. if your temps spike over 62C during a prime run your cooling isn't adequate for your overclock.
understand, with the deneb, the cooler you can keep the chip the lower the vcore you need for any given overclock. this is a very sensitive cpu to temps. My deneb is stable up to 58C, i can run prime all day if i want. but if i get my room hot enough, and here in phoenix just turning off the ac will do this durring the summer (40C ambient? yeah... can't run prime without ac in the summer here), my chip will hit 58C and it will crash or lock up. (usually inside of seconds)
Also understand that denebs don't overclock linearly. there comes a point you get diminishing returns from your overclock. with a 955 i'm pretty sure that point is somewhere around 3.6ghz... once you kick it over that point, you get drastically reduced performance gains from the clock bumps. for the 965 that point is somewhere around 3.8ghz... when you get past those "sweet spots" you gain more performance boosts from overclocking your northbridge and ram... tightening the ram timings or something, then you do from higher clock speeds. I know i saw very little improvement in my benching results with clock speeds over 3.8, but when i tightened up the ram timings and increased the nb frequency i saw almost x3 the gain i got from the jump from 3.8ghz to 4.0ghz. in short it was much more efficient to keep my chip at 3.8ghz and boost the northbridge and ram as much as i could, then it was to stretch out the cpu frequency and pour power down it's throat.