Whats the difference between gtx650 & gtx650ti?

The GTX 650 Ti's GPU is about twice as fast as the GPU of the GTX 650 and the GTX 650 Ti's memory is clocks higher. It is worth the extra $40, but better than the GTX 650 Ti is the oftentimes similarly priced Radeon 7850 1GB and the Radeon 7770, although a little slower than the 650 Ti in most situations and more than a little slower in some situations, is generally a better value because it's usually almost as fast as the 650 Ti, yet only a little more expensive (if at all) than the GTX 650.

Your budget is definitely enough for a Radeon 7850 which is the best option within your budget. If you must have a Nvidia card, the the 650 Ti is your best option, just keep in mind that the 7850 is a faster card.