If you're a gamer, you should not buy a pre-built machine, especially anything from eMachines, Compaq, or HP. I believe all pre-built PC manufacturers have a clause where you void the warranty if you open the case yourself. If you ever want to do upgrades yourself, custom-built is the only way to go.
Besides, if one part goes bad in a custom-built, you can ship it back to the manufacturer. If something goes wrong with a pre-built, say bye-bye to your PC and hello to extravagant repair or shipping charges.
NOTE: I'm being really restrained in this post. Those who've discussed pre-built machines with me in the past know how adamantly I argue for building your machine yourself. There are, of course, situations where pre-built machines are not a bad choice. Those situations are very few and far between, at least for home users.
