hey,
Personally I would get a gtx670. If you mean $330 for one gtx560 then I would definitely not go with that. A gtx670 is $~370 depending on where you live. Two gtx560s may offer the same a bit better performance in a game or two, but you could encounter driver issues, scaling issues, micro-stuttering, and a bunch of other potential problems. So overall you could very well get lower performance with to gtx560's. A gtx670 is a very good card and it should handle any game you throw at it at more or less Max settings. Also in the future you could just add another gtx670.
Hope this helps!