Building gaming computer

[mxplaya]

Hi all, this is my first time attempting to build a computer, and could use some advice on a build for mainly playing WoW, my budget is $600-$650.

[GroundZero3]

Do you need a monitor, os, keyboard and mouse, or case or do you have that already?

Do you prefer ATI or Nvidia
Do you prefer AMD or Intel?

[mxplaya]

have monitor, keyboard and mouse already, need everything else, and really dont have a preference, never really added anything to my computers from the past, but they have all been intel and nvidia