For the low to mid-range computer, it is almost always cheaper to buy a Dell. There is no way any single person could match the volume discounts they get. When you get into the high end though, things change, and it usually isn't too hard to build one cheaper than Dell or Alienware.
ORAC's suggestions look good to me, although I would try for a 3GHz (or AMD Athlon 64 3200) or faster processor and 1GB of RAM if you want to use FlightSim on it one day. Strategy games are getting more advanced graphically, so even they would benefit from something in the GeForce 6600GT range.