Yeah the 660 non-ti is still a beast compare to 650ti or 7770 so you'll be able to play no problem to all games with high to maxed out detail at 720p and high at 1080p (high at 1080p looks a lot better than 720p maxed out...)
720P = 921 600 pixels on the screen when 1080p = 2 073 600 pixels... More than the double so expect about half the FPS at 1080p over 720p unless you we're CPU limited at 720p (less resolution tends to stress more the CPU and the inverse is real too...)
Half the FPS is still playable as the standard for gaming fluidity is only about 30-40FPS so if you get more, you're ok to play and 1080p look awesome comparing it to 720p... you'll never go back when you'll start playing at 1080p...