Well my advice would be upgrade your power supply firstly. A XFX, Seasonic, Corsair, or even a decent Antec should do the trick. Then I would buy a mid range card meaning the 2nd number of the card
650, 650Ti, 7770 being the extend of what I would recommend based on your CPU as core 2 duo's are 4+ years old now.
How many watts should I go for for the power supply