You do NOT want to be using generic 1333 CL9 RAM as you are bottlenecking the entire setup and losing a large chunk of performance on the A8 APU, which is very bandwidth-intensive. Look at MINIMUM to a DDR3-1600 kit. There is a Team kit that costs less than $38 shipped and is DDR3-1600 with lifetime warranty.
With a $500 budget, you should look to avoiding the APU together because you can get something better. Look for an FX-41XX or Phenom II x4/x6 (1045T is just $133 shipped on Superbiiz!) + HD 7750 or 7770 or 6870 PC.
(Oh yeah, did I mention that the HD 7750 outperforms the 550 Ti at half the power consumption and for less price?)