For $550, you did pretty good. The GTX550ti * will give good FPS up to 1680x1050 resolution if you back off on some of the eye candy. You could save a buck or so by going with 1333MHZ memory, because the PhII memory controller tops out at 1333MZ. You have to O/C the RAM to get 1600MHZ.
* I just sold an AthlonII X3 455 build with a GTX 550ti. It did surprisingly well. The only game I had to back off from 1680x1050 was Deus Ex: HR.