Well this is a oldish post and not sure if you got one already,but what exactly do you mean with "gaming PC".If you go to a store and say you want a "Gaming PC"They will give you a mega pc.Few things you do need to keep in mind is CPU,RAM and GPU.These are basics that you look at first.Then smaller things can improve your gaming like:HDD bit speed,PSU size,RAM bus speed.Even your OS has a effect on your gaming experience.
So basicly you can take a mid-range PC and make it a "Gaming PC".So check the games you are planning to play and compare them with a new PC's specs.