for 210$ u can get an HD7870 which will play these games very easily,
and las a lot longer than GTX 650 ti
by the way what are D3 and SC2?
It will cost him around 300$ since he needs a new powersupply. My choice is logical with what games he is playing and the rest of his system. If you have any money left over consider getting 8 gigs of ram. I eerily have a very very similar system.