these aren't desktops. They're far from upgradeable at all. Most if not all GPUs are soldered into the MOBO and can't be replaced, very few CPU's can be switched out, but even then a change of socket type will occur in the next year or two, so that's also useless. And Changing the motherboard to even a knowledgable user is extremely difficult.
$500 barely buys you a decent laptop for general work.
What games are you aiming to play? and at what setting?
found this on ebay, refurbished but a solid starting point:http

/www.ebay.com/itm/Lenovo-G505s-Gaming-Laptop-PC-AMD-Quad-A8-5550-4GB-500GB-ATI-HD8570M-Video-/151329625477?pt=Laptops_Nov05&hash=item233bf2d585
if you're willing to go $100 over budget then its very worth it, this machine hasn't been released yet but the specs are solid for the price point: http

/www.amazon.com/Acer-Aspire-V3-572G-587W-15-6-Inch-Platinum/dp/B00K81O3PY/ref=sr_1_3?ie=UTF8&qid=1402952932&sr=8-3&keywords=840M+acer