You get a better computer.
Takes about 8-10 hours of research into what you're going to use, then you have to order it and assemble it, and if parts don't work, RMA them.
You also have to get your own operating system, which adds cost as the OEM versions of windows are about $120-150. However, you can use BSD, Lunix or Haiku and they're free.
Stick with major brands and stuff that's 18+ months old. You won't come into conflict with this rule unless you want a really cheap computer (under $400) or a really cutting-edge (over $2000) one.
If you just want cheap, your $400 Dell is better than what you can do -- but for another hundred bucks, you can get a much higher-performing machine with none of the goofy.