though I'm actually not too familiar with phenoms (was young and naive when they were popular), I will assume so, since chips of the same family are usually manufactured exactly the same and the name labeling is just an indication of how well it's binned and what sorts of factory tweaks it receives.
If you have a poorly binned Phenom II x3 and unlock its 4th core you may not even get to 3.4Ghz, but if you can I'd assume that it'd be similar or the exact same as a phenom II x4 965 at stock, however, it is as you suspect, since the 965 is a factory 4 core (unlike your x3) and is binned higher (clocked at 3.4Ghz right out of the box) it has a better chance of reaching farther clocks than the 720. There is a soft ceiling of sorts with any sort of chip though so it may only get about 200-300 Mhz above it.
If I grab say an AMD Phenom II x3 720 BE, clocked at 2.8ghz, and then OC it to 3.4 ghz,
Will that run identically to a Phenom -- 740 clocked also at 3.4 ghz?
Oh and also, if I unlocked the 4th core and OC'd the 720 to 3.4ghz, with 4 cores now, would it be identical to a phenom II x4 965? Or does the 965 OC much farther?
Ok...here's the skinny.
OC'ing will negate differences in chips within the same family. It will also help to keep older technology relevant longer.
Unlocking a 4th core on an old 3 core Phenom would make it equivalent to the X4 version at the same frequency.
HOWEVER, in the past, not all people who successfully unlock the extra core can overclock them very far. This was a sort of luck of the draw type of thing. I have seen them unable to be overclocked at all and stay stable, on the other hand, I have also seen them overclocked more than 1 GHz with an unlocked core. So, just keep that in mind before you move forward.
thanks guys! i found a phenom x3 720 for 30 bucks so might as well give it a go. theres a x2 b59 that starts at 3.4 ghz and can maybe go to x4 for 25, so maybe ill just pick up both and compare! thanks again
p.s. - both answers we're helpful, i went eenie meenie minee mo to pick best answer