I can't give you an exact percentage but VGA plays the most important role,(lets say about 70%-90% depending on the game),the its the CPU and RAM.
Overall VGA has the most impact on gaming performance.
I'd have to agree but would like to add the hard drive system since most graphics load the video card directly from the harddrive with minimal cpu time a fast HDD system will make a noticable difference. I'm not talking about going from 5400 rpm drive to a 7200; you'll see the boost when you go from a single drive to a raid array or an SSD.
how much each component affects the fps in like a %
Once the game/map is loaded into ram, the harddrive play 0% in the FPS. None. The GPU plays the biggest part, followed by the CPU and then the ram. The drive system only factors into loading the game or maps. Once that is down it play no part.