Upgrade computer/ new build $300

I have a computer right now and its getting pretty old so I want to build a new rig. I already have a new case for it, the antec 900. I want to just place my old hard drives in this new rig and also would like to reuse my 550W power supply and the optical drive. So I need a new cpu,motherboard, ram and a gpu. Does anyone have any suggestions as to what parts I should get for this? My budget is $300, but I'm willing to up the budget to $400 if it gets me something much better. 

What do you intend on doing?

I intended in using it for gaming, just average gaming