What component upgrade would I benefit from the most for the money?

[D2junkie]

First time posting long time lurker, just looking for some opinions and insight on what would be the most performance increasing upgrade for my current system.
Thanks

Processor: AMD Phenom II X6 1075T 3.00 GHz
RAM: (4 x 4GB) 240-Pin DDR3 SDRAM
GPU: Nvidia GeForce GTX 550 Ti 2GB

 

[tiny voices]

Graphics card. 100% guaranteed.