What would be the most substantial upgrade for my system?

Lookin2Upgrade

Honorable
Sep 18, 2013
10
0
10,510
Currently have:

AMD Phenom II X6 1055T Processor 2.80 GHz Watercooled heatsink/fan

ASUS M4A79XT0 EVO Motherboard

4GB RAM

ATI Radeon HD 5800 Series PCI Express 2.0 1024MB GDDR5

64bit Windows 7 Home Premium

(NEW) 250GB SSD Hard Drive Running OS

2x 1TB Hard Drives for storage

(NEW) 750W Cooler Master Power Supply

My system is 2-3 years old and I am looking to upgrade whatever will make the biggest difference for online gaming.

Not sure if upgrading the graphics card is the best idea, or if I will need a new processor and motherboard to catch up to modern graphics card requirements.