If you keep waiting you'll never be happy. Plus, what tells you the new nvidia cards will be cheap? They'll perform better than the 6xxGTX but they will cost more too. So just buy the 670GTX and SLI it later as Estacado suggested. Just be sure to have a good PSU if you're going to SLI later. I should mention though that not all games scale well when you have SLI cards, for example, you'll probably get 50FPS with 1xGTX670 in a specific game but when you add the second card you'll probably get 70FPS. Some games are more optimized for SLI cards than others, like BF3 for example, it's going to pretty much give you 90% scaling performance (i.e. you were getting 50FPS, with 2 cards you'll get 90FPS and so on). If you don't want to deal with SLI problems and your games not being optimized enough to make use of 2 cards then just get a GTX Titan :3
It'll serve you for another 5 years at least.