I just got my Rosewill 550w psu from newegg. And I noticed that it had an option extra 4-pin part to the mother board cord.. So i guess that means you could choose to have either 20 or 24 pin? My question is.. do i plug both in? why is there an option? my old one had only 20pin one and there were 4 empty slots. I guess before I keep ranting. What is recommended and why? and so on! Thanks