Okay, so I have been in a relationship for 2 years and my boyfriend's ex used to be my best friend. Well now we are friends again, she comes over, and visits while he is here. Last night they were being soooo talkative, and if you knew my boyfriend you would know that he never talks!

They would sit really close, and once she even acted like I wasn't even there! I want to break up, but I don't what to do. I just hate the way he looks at her : /

You need to talk to him, ask if anything is going on again. If you want to break up with him do if you don't trust him. It would be very difficult to stay with someone you don't trust.
Ask your girl friend what's up. Tell her you don't appreciate the coziness she is showing toward your boy friend. If she is a friend she will stop.

Indeed, you should calmly tell him that their good relationship makes you feel uncomfortable and left out. There is often lingering attraction between exes. Unlike with friends and acquaintances it's not selfish to be possessive in these circumstances.

Talk to him, and tell her you want some alone time with YOUR boyfriend. I mean who lets an ex visit while the boyfriend is at YOUR house in the first place?

How old are you guys?