I need a PC component upgrade

[Ming Le]

My PC specs are
CPU: AMD Athlon II X3 440 @3.0GHz
GPU: NVIDIA GeForce GTX 560 SE
RAM: 4GB @1333MHz
HDD: WD Green 1TB
OS: Windows 7

What should I upgrade for maximum performance?
I can only afford an upgrade for one component.
The goal is to run Warframe at moderate to low settings.

 

[Skyl3r]

Are you using a 64bit OS?
If so (or even if not...), then you should go up to 8GB of RAM. That's probably what you're farthest behind on as far as current standards go.

If you give some sort of price range, it'll be easier to tell you how you can maximize your benefits within your budget.