If u want to play at 1080p, 7850 is ideal. 7870 recently became cheper so it is also a top purchase. 660Ti is stronger than 7870 in some games and weaker in some others, however is quite more expensive than 7870 (its is supposed to rival 7950, however me as well as many other forum users doubt it).
7950 is the next step, it does not worth the extra backs, but always superior performance comes with a price. 670 is above 7950 and below 7970, but you should consider them only if u play on res higher than 1080p.