If u want, you can visit my site
www.gamputer.com.
The design is plain, but I assure you once you are done reading the content, you will be pretty good about knowing a good gamputer from another (or if a computer is for gaming at all).
In general, the most important components of a PC are
Motherboard-All the hardware components are added here.
CPU-The Intelligence of your PC, the performance speed of processors determines the speed and ability of your computer to take care of tasks (eg, games!!).
RAM-Short Term memory of your PC, need for storing short term data (eg, website, games). Good RAM and CPU=Your PC can have many applications open at once smoothly (eg, 30-40 websites with movie player, microsoft word, photoshop...you get the point).
GPU-Communication skills of your PC. Good GPU is a must for gaming computers. No matter how good your CPU and RAM is, without a good GPU the PC won't even be able to run the game (If it is below the game's requirements).
HDD-Long term memory (stores files etc) decent speed is necessary so that the computer's performance speed is enhanced in general.