A 4650 would most likely run with your existing PSU, but not the 4670, it uses more power and migt be just a little to much.
Nvidia options are;9500GT, a little slower than the 4650 and uses a fraction less power or the 9600GSO/GT, faster than the 4670 but uses more power.
My advice would be to get a 350-400W supply from OCZ, Corsair, Seasonic, Antec or PC Power and Cooling and go for the 4670