Everyone rushing to conclusions...
briggs, why ask this question? Are you experiencing any difficulty in certain applications or games? What do you do on your PC on a day-to-day basis? You can't pinpoint a bottleneck with the components you have, bottlenecks will differ with different applications or use. Especially since all your components are decent(ish). If you had an Pentium 4 with a GTX660, then there is a clear indication where that will be, but not in your case.
Also, it will be a lot more helpful if you could provide us with a full list of components (with model names).
If you are a gamer, it is most likely the CPU. The GTX660 OC will still be able to punch the numbers on 1080p without breaking too much sweat.
If you are just getting frustrated with how slow your software loads (Windows, MS Office, etc.), it is more linked to the HDD. Get an SSD and install Windows and your programs onto the SSD, it will make a massive difference in using those programs and loading Windows. If you don't like the idea of an SSD (or don't have any cash to spend), then do a format and reinstall Windows, without all those extra programs you don't use... <-- This won't make much of a difference.
If you are doing more specific things like Video editing or Graphic design. Then we are talking a different ballgame.
Are you looking to upgrade a certain component to improve the PC performance for your intended use? If so, how much are you willing to spend (your budget)?
Upgrading you CPU will force you to upgrade your motherboard as well, which is a good thing for all the extra features you can get. For what you have at the moment, the GPU seems like the only component not to upgrade on that PC, and the RAM.