Hi J1,
Yes, you should have no trouble going to an i5. Dell-Chris M listed these as compatible with the 660:
972H5 3.30GHz i3-2120 Sandy Bridge, 3MB, 65W, Q0
JN8M2 3.10GHz i5-3450 Ivy Bridge Quad Core, 6MB, 77W, E1
Either should work fine with your power supply.