The Phenom ii x2 550 might unlock to 4 cores. And might run stable. And you might be able to overclock all four cores and still run stable.
But you'd have to get lucky 3 times for all that to happen. Once for each core to unlock and run stable and once for the overclocking.
Power efficiency doesn't make a chip better, it makes it more efficient ant thats it. On performance a X2 is very old technology and is hence slower than the i3, At the pricepoint and for not much extra a X4 960T Zosma can be aquired which is a quad with potential to unlock to a hex core. If you are able to go that route then it will be better than a i3 route, if not then get a I3 paired on a H61 motherboard, low cost and decent performance