OP, I'm not sure you realize how vague of a question that is. There are a ton of factors to consider - is this a pure gaming machine? What is your budget? What sort of knowledge do you have? Is an off-the-shelf box ok, or do you want to build it yourself? If you want to build it yourself, do you have a good reason?
Your budget really sets the bar for this whole thing - you can build a pretty good gaming rig for fairly cheap these days if you have low requirements - especially with cyber saturday coming up near the end of the month.
Have you ever built a machine before? If not, you may want to consider buying something off the shelf from a major manufacturer, or a reseller. The reason is that its much harder to build a machine for cheaper than an off-the-shelf equivalent these days than it used to be. Usually I advise people against it unless they have a really good reason for it (wanting to know the quality of each component, etc). Plus with off-the-shelf you can get a warranty and support if something goes wrong. If you are building it yourself, you're the only person you have to rely on.