Yea even if your a bit tech savvy, everyone should still try to build there own. It'll be about 1/3 of the cost those places will charge you.
an app a thread for that.
Plaes like ibuypower, cyberpower, and tigerdirect do charge more than it would cost to build it yourself, but not nearly as much as prefab retailers like Dell, HP, emachines.
For a non gaming, "everyday use" system, a retail machine from Best Buy or Walmart will probably be fine.
For a gaming rig though, building it yourself is cheapest, getting an actual custom built machine from a company that is not a major manufacturer is next best, and getting a system from a major manufacturer is the worst, both in terms of price and buyer control.
Again, more detail on all this in the tread I linked.