I like how no one concretely answered his question and are recommending cards some $80 more expensive than the $220 660.
Anyways, 550 Ti vs 660. Survey says:
http://www.anandtech.com/bench/Product/541?vs=660 more than twice the performance.
At about $220, you have two options: 660 or 7870. In BF3 and WoW, the 7870 pulls ahead slightly with Catalyst 12.11 drivers. In SC2 the tables are turned, the 660 dominates the 7870.
You have to keep in mind that Nvidia's releasing the beta driver 310.33 soon. They also boast performance boosts so the $10 price difference of the 660 might actually be justified.
Here's an article published by Nvidia that boasts its performance increases with 310.33:
http://www.geforce.com/whats-new/art...ivers-released