Long Term DDR3 RAM upragde for Phenom 955 system

I have a build and I am looking to upgrade the memory, and I am shooting for the most long term upgrade I can get. A little about my system:

Phenom 955 4 cores and 3.2 GHz
ECS a790 DDR3 (240 pin RAM)
4GB DDR3 1333 Patriot RAM
4890 Graphics card
Seagate disk HD 500 GB


RAM is so cheap right now and mine is holding my system back, but I want to upgrade for the long haul, so what are my best options? 4GB of corsair 1333 RAM is only 23 dollars right now, is that where I should go? On my task manager I am usually using around 3 GB of my current RAM (I have never been over 3.5 GB usage)

I figure If I start with 4 new GB I can try it with my current RAM and see if they work well together, and If they don't I still think it will be an upgrade and I could get 4 more GB of the new corsair.

What do you think