Upgrading my system

I'm not a knowledgeable person when it comes down to computers.

I want to upgrade my computer this year for casual gaming.
I have a video game console, so i don't use my computer for the hardcore games.
I just want to know what i can buy to upgrade my system with a budget of about 400$

OS Version: Microsoft Windows 8, 64 bit
Processor: AMD FX(tm)-6300 Six-Core Processor, AMD64 Family 21 Model 2 Stepping 0
Processor Count: 6
RAM: 7898 Mb
Graphics Card: ATI Radeon HD 4250, 256 Mb
Hard Drives: C: Total - 953866 MB, Free - 909959 MB;
Motherboard: BIOSTAR Group, A880GZ

Edit: I have a 550w power supply
Model #: ATX-550

Thanks