For $650, for 2 new non reference gtx670's is a nice deal. You will have no problem playing the latest games on ultra, and depending on what game it is yes, your 120hz monitor will take advantage of the higher FPS. Two gtx670's in SLI are beasts!
Check out this review of two of them:http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_670_SLI/
Just make sure your power supply can handle two of them.
Hope this answers your question!