I'd save up until you can afford a new motherboard and CPU. Yes, I'd shoot for i5 in your situation.
The AMD llanos basically a the same design as the Athlon II and Phenom II but with a graphics chip on the die. The difference between A IIs and P IIs is that the A IIs don't have an L3 cache (neither do Llanos), depending on the program or game this can affect performance anywhere from 5 to 25 percent. So yea, the 660 TI is not the best pairing with a Llano, they work best as low end systems that don't need discrete graphics cards.