3.4 with 2 volts is the same as 3.4 with 1 volt, it just depends on what is stable. the 2 volt one would get a whole hell of a lot hotter than the 1 volt though.
if you work with the FSB, however, then things start to change a little bit.
p.s. DON'T pump 2 volts into it unless you need it, which even then, i would tell you not to.
ok, lemme rephrase to make sure i got it right. if you can get 3.4 with one volt, awesome. if you can get to 3.4 with two volts, ok. they wont perform differently, unless a few years of life extra is an absolute must. you can set voltages manually.