Upgrading a 2 year old store bought PC

What will you be doing with this PC? Be as specific as possible, and include specific games or programs you will be using.

Gaming and potentially doing school work.

What is your maximum budget before rebates/shipping/taxes?

$800

When do you plan on building/buying the PC? Note: beyond a week or two from today means any build you receive will be out of date when you want to buy.

Within the next few months. Looking for advice and recommendations.

What, exactly, do you need included in the budget? (Tower/OS/monitor/keyboard/mouse/etc)

Advice on internal components. OS, keyboard, mouse, tower, etc. already purchased, though if a new tower is needed then I would appreciate a recommendation.

Which country (and state/province) will you be purchasing the parts in? If you're in US, do you have access to a Microcenter location?

US, Ohio. The closest Microcenter is an hour away at least.

If reusing any parts (including monitor(s)/keyboard/mouse/etc), what parts will you be reusing? Brands and models are appreciated.

I will be reusing the monitor and any other peripherals I have. I may potentially reuse the tower but that will be replaced if the new parts cannot be incorporated into the old monitor.

Will you be overclocking? If yes, are you interested in overclocking right away, or down the line? CPU and/or GPU?

I will not be overclocking at the moment though I may consider it in the future.

Are there any specific features or items you want/need in the build? (ex: SSD, large amount of storage or a RAID setup, CUDA or OpenCL support, etc)

There are no specific features I want beyond playing games at a reasonable FPS and settings.

Do you have any specific case preferences (Size like ITX/microATX/mid-tower/full-tower, styles, colors, window or not, LED lighting, etc), or a particular color theme preference for the components?

I do not.

Do you need a copy of Windows included in the budget? If you do need one included, do you have a preference?

I do not.

Extra info or particulars:

I already have an OS and other software as needed. Beyond replacement parts, I just need to know if the tower is worth replacing. My PC is an HP Pavilion 500 with an AMD elite quad-core A8-6500 accelerated processor, an 8GB DDR3 system memory, 2TB hard drive, AMD Radeon HD 8570D Graphics card, and a dvd burner and wireless LAN 802 11b/g/n for what it's worth.