What's important in a gaming computer?

I watched a video on YouTube created by "TigerDirect" where they created a gaming computer just for 479.99 or 469.99 before rebates by shopping for the parts and making it on their own instead. I feel stupid, I honestly should of really just tried to make my own computer... I would have spent the same amount for this prebuilt one where I could of spent the same amount of money building my own, beating the crap out of the prebuilt.. anyways just go on YouTube or Google, gaming computers less than 500 bucks. They are what you call a good starter kit ;), and you can go on from there and upgrade on your own and etc.