To be honest, you have a great system as it sits right now.
If you look at this chart:
http://www.tomshardware.com/reviews/gaming-cpu-review-o... your CPU is still on the top tier.
Your graphics card is pretty good too.
http://www.tomshardware.com/reviews/gaming-graphics-car...
The only thing three tiers and above (the recommended difference for an upgrade to be noticeable) is GTX 590, Titan, GTX 690 & Radeon HD 7990.
What i'm trying to say is... i don't think that an upgrade to a GTX 670 will make enough of a difference for you to really notice. Unless silence is your objective, and not power... then your better off spending your hard won money on something else.
Either get a faster graphics card, or invest in something like this:
http://www.wegotserved.com/2012/07/03/building-windows-...
I have a little atom PC that i use as a server (granted mine has Windows server 2003) for all my movies, music, ect. and its been the best investment ever (its also great to use as a VPN Server
http://gizmodo.com/5990192/vpns-what-they-do-how-they-w...)