$500 prebuilt isn't going to get you much. Besides, you really shouldn't buy from Best Buy (Circuit City is no longer in business) as I can only assume that they load their computers up with as much junk that's either spam or as close to adware as they can get without it being illegal.
Order a Dell or something online. You can get a Dell that does passably well on most older games (including all Source games) but recent games are more or less out of the question. I had a $450 Dell that the best game it could run (even on more or less the lowest settings) was Assassin's Creed 1.
Even a >$500 built computer isn't going to leave you much wiggle room, as there is more or less a minimum you have to spend with a prebuilt to get something that isn't going to be stuff quality, let alone play games well.
