what power supply do you have?
chances are it wont be enough to go for a 430/450, old pc parts arent known for being economical, and power supplies are usually the first bits in a pc to die.
and if you're considering a 450, might as well go for a 460
with the specs/age of your pc, its pretty much advised to replace, well... everything, your pc as it is, wont be able to handle bf3 too well, even if you do get a graphics card
i3 or i5 is probably the most recommendable for gaming, but they have a bit of an attached price tag.