Wow there is a lot of argument in this but I think that you will find that boxes already setup and advertise as gaming PCs will have what you need as a guide.
Thing is the prices and options vary so much with gaming computers so basically what I did was look up the games I wanted to play and the system requirements.
When i buy a game it tends to last me a few months to even a year so I don't worry about hardware changes plus you get other titles cheaper because everyone else is anxious for the newest games.
Me too! I notice some like to play 3-4 games at the same time doing a little bit on each but i can't enjoy it that way. It might work for arcade stuff but not serious games.