What's the difference

whats the difference between geforce 650 and 650 ti 

Power; the 650ti is a tiny bit better than the 650. Both are relatively poor cards, and won't do too well for much of anything other than daily office use, but the ti is the better model.

whats the best card i can do for around $150

I think the new 7790 is a good card or a 7770 if you can't find a good deal. or a 650ti if you want nvidia.