Pre built computers are generally cheaper, but its because there are cheaper quality parts, and parts that don't run as fast as its counterparts you can buy separately. However, if you are going to use it for basic computing and fairly simple gaming, at a budget, I would go with one of these.
Building your own computer, you can pick your own parts, at whatever price (so to speak). Although the price of building your own computer has significantly come down in the last decade or so, it's still cheaper to buy a ready to go out of the box pre built one. If you were going to get one for more high end gaming, I would go with building your own, since you know you won't get skimped out of parts that you might need. But it generally cost more. Also, If you build your own, there's no support on the computer as a whole, so if something goes wrong, you ll need to narrow down which part is giving you the problem and seek support from there.
I wouldn t buy anything from best buy. Everything is overpriced. You should try Frys, or online such as.Frys, or newegg, tigerdirect, etc.