Power hungry things are
intel core i3-2120 (Planning on getting this)
a seagate barracuda 500gb 7200rpm hardrive
and an ATI hd4670
gonna have a 2X4gb ram setup also with a Coolermaster 212 plus evo
Like I said I only have a 300W psu when I get home I'll tell you the brand and model, but I want to know if I'd have to upgrade these things to even get it to work
I've been using an AMD X2 6000+ that came with the pc so it should be easy for the same psu to power a new sandy bridge processor right?
Yes that 300w PSU should be just fine. Efficient? Maybe not, but that depends on the make, model, and rating of the PSU. Your GPU should consume around 59 watts (peak) source
and your CPU around the same. More importantly I wonder what your amps on the 12+ rail are. But even that I'm sure will be enough for your components.
Those are the two most power hungry components in your computer (CPU and GPU) and those specific ones aren't that power hungry at all =). So for the current set up you can use that 300w PSU.
Good luck mate.
PS: Oh, and yes that sandy bridge will use much less power than the amd x2 6000. =)