Building off my Gateway DX4840


Dec 12, 2012
Hey guys I am new to building a computer and want to build a gaming computer. I recently picked up a Gateway DX4840 for a good price and want to upgrade it further. What should I replace/add toamke it good. I want to run games like Guild Wars 2, Fallout games, Battlefield 3 Crysis, Farcry etc...
In my Pc I have currently.
Graphics: ATI Radeon HD 5450
Processors Quad Intel core i3CPU 540 @ 3.07 Ghz
Motherboard: Intel® H57 Express
I believe.
I would like to keep it below 800 if possible and be able to run games close to high settings if not max.
Thank you for responses