The answer is unclear.
To help clarify your options, run these two tests:
a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 50%.
This will simulate what a lack of cpu power will do.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
set to 50% and see how you do.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
If you need a graphics upgrade, make it a big jump or you may be disappointed. Your GTX550ti is a reasonably competent card.
I would look at a GTX650ti boost/7850 class card at a minimum. Your 600w psu can handle a card as good as a 7970 or GTX780.
If you are looking at a cpu upgrade, it will be because the FX cores are individually too slow. A ivy or haswell overclockable "K" quad would be in order You are then looking at a motherboard change too.