The better the graphics card, the better you will do.
You get higher FPS, more eye candy, and better minimum frame rates.
But the stronger you go, you get smaller improvements for your dollar.
Buy the best card you feel comfortable paying for.
Make certain that it is much stronger than what you now have, or you may be disappointed.
Use this tom's article on best gaming graphics cards for the money(august 2012) as a guide:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
Prices will have changed, and the newly launched GTX660ti @$300 is a blockbuster.
With your 550w psu, you can run almost any 28nm based card up to a GTX680($500).
28nm is used in the GTX6... and amd7... series. I would stick with those.
To get an idea of how well you might do, 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 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.