965 is able to operate at higher clock rate than a 955 at stock settings. that's why amd charges more for it. you can buy a 955 be and oc it to the similar level as a 965 be. it's called product binning. same thing with phenom ii x6 cpus. you can save money by getting the 955 be.
however, every cpu is different and overcloking depends on luck. two cpus from the same series might have stable oc at different settings.