Why Spend More?
That's it. Everything needed to build an outstanding gaming computer is listed here. Again, it is important to stress that this will be an outstanding gaming PC. It will handle nearly anything thrown at it. The only games which may slow it down will be Crysis (duh) and the recently released Arma II, which is not that attractive but is still demanding. The total price of everything listed in this build will cost, in total, $690 dollars based on pricing from Newegg as of August 2009.
This might be hard to believe at first. Companies like Alienware, which are dependent on the price of gaming computers being high, have done a very good job of making it appear as if a good gaming computer should in fact cost thousands of dollars. That used to be true. But hardware has gained power more quickly than games have found ways to use it, and the importance of porting new games between the 360, PS3, and PC has also put the brakes on graphical advancement. The large developers which have the money to spend on the graphics see no reason to push the power of the PC because the PC version will usually sell the least copies, and major advances on how much hardware games can use will have to wait for next-gen consoles, which we've heard little about.
Spending more money for better hardware will increase performance, but this build is what I consider to be the perfect gaming computer. Expect diminishing returns. A computer twice as expensive will be nowhere near twice as fast. In fact, its performance will be visually identical in most games. There is no way to turn the detail settings of a game higher than maximum, after all.