All right, there are a few variables here.
what kind of games is your son going to be playing?
if he's like most people, when you get a gaming computer, your going to want high graphics, NO LAG, fast, smooth, cool case and a really powerful cpu and gpu.
That's great, but you're looking for something inexpensive.
as far as in-expensive goes, the bear minimum involves the owner building their own computer. I've heard that that usually costs around 400$ as a minimum (without monitor).
if you live in the U.S. then your in luck. you can go to the "cyberpower" website and (after doing quite a bit of research on the most important parts of the computer) you can use a guideline build to costomize and build your own gaming computer. they will build and ship it for you.
the most important parts in order from most important to least important are:
gpu (graphics/video card) - the most important component for gaming
cpu (processor) - also important
ram (memory) - at least 4gb
hard drive (long term memory) - you'll want 750gb - 1tb at a fast rpm speed.
this will probably cost 500-600$ (without monitor)
if you want to go the super easy route, you can go to an online retailer (future shop or best buy are examples) and buy a stock gaming pc, usually cyberpower. this is ok, but usually the stock gpu's are a little sub-par and may need some in-game graphics adjustmants to be playable. if you're not looking for great graphics, but you want a gaming computer, this is probably the way to go. if your son finds himself at any point wanting better graphics, these things have lots of extra space, so you can install a new gpu (which is fairly easy with a good tutorial) for about 140$ (the cost of the new gpu).
a good example for a gaming computer like this is:
http://www.bestbuy.ca/en-CA/product/cyberpowerpc-cyberpowerpc-gamer-ultra-desktop-computer-with-amd-phenom-ii-x4-processor-955-black-edition-gua140-gua140/10173254.aspx?path=4aff7cd386da9aad5b41d05f6fa76888en02
(this is also without monitor)
this one costs 570$ and you can go a pricey-er if you want.
I hoped that this all helped.
this website is a great place to have questions like this one, as well as any other questions answered.
that is just my opinion.
don't buy this soley because of what i just said, get some other opinions as well.
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