GTX 650

the GTX 650 performance is ok for a mid range system, you'll probably be able to play most games with it at medium-high settings (apart from some new/demanding games). the performance is way behind a GTX 560 though.

The biggest advantage the 650 has it that it only uses 65watts under load vs 150watts used by the 560

the 560 is much better at handling higher resolutions and is about 35% more powerful then the 650.

If you look around you may be able to pickup a used GTX 560 for about the price of the GTX 650 (well in the UK I found some in the same price range as a new 650).

If you're building a system from scratch you may be able to save money with the 650 by getting a smaller power supply but don't forget that this also limits your ability for future upgrades.

The GTX 550ti is better than the 650.

The big advantage is the power draw, it's about half of the 550ti. It's not even a fair comparison when comparing the 650 and 560.