A good gaming computer is mostly defined by the performance of its hardware, but also by customer support. Whether you're buying a pre-built computer from Dell or HP, or building your own, knowing the possibilities is a must.
AKA "bang for your buck." Unless you have a ton of money to spend, you want to get the most performance for the money you spend, while not sacrificing support options.
The Central Processing Unit is the brain of your computer. Intel's Core i7 chips are faster, but AMD's Phenom x4 CPUs have better price-to-performance.
Most system builders won't let you choose your motherboard. If the option arises, Gigabyte and ASUS are popular choices, with Gigabyte offering the best bang for your buck.
Power Supply Units
Don't get a cheap PSU. PC Power & Cooling and Corsair offer PSUs that deliver steady voltages and are designed to more efficiently and quietly cool themselves.
You don't need more than one video card in your system unless your display screen is about 30 inches or larger. You also rarely need the fastest video card, which always comes at a steep price premium.