It's very complicated and it will take me ages to explain everything, but essentially this is what you need being put in the simplest way possible...
internet connection - modem - router - switch/hub - network devices (computers, printers, etc...)
ehhh... 144-60 = 84 you will see up to 84 more frames, if your GPU can produce that many depending on the game.
whether or not you will notice that, is really up to you, some people will whilst others won't as much.
Your system is well balanced, you don't really need to upgrade anything unless you doing some specific where you feel that you would benefit from an upgrade. What do you expect to get out of an upgrade?
You could cheap out on the case, optical drive, RAM (in most cases), motherboard, and possibly the PSU (not that I would recommend doing that though) whilst maintaining most of the performance.