Ah it's not that simple. Much of the time they will keep you around much longer than necessary until they are detached from you and can make a transition to a new guy more smoothly. I would be concerned with this because I have read stories on this very forum about guys that have lost their girls to other guys on XBox Live. It progressed from game friends to talking on the phone and so on and so forth.
Unfortunately the snooping just made things worse. It gives her some ammunition against you to keep herself validated in what she is doing right now. Communication is key and that was a break down of it. It's true that she probably would have denied it anyway but it takes that as the first step.
Anybody that plays XBox that much is living an unhealthy lifestyle. Being a former big video game nerd myself I can attest to that. Her life has to be suffering in other ways too. Clearly your relationship, but what about her work, school, physical health? Having a talk with her about this and opening the lines of communication is probably your best bet in the beginning. You don't have to accuse her and say this is what needs to be done, but maybe ask her some questions. Why is she doing this? Does she think this is right? Where does she see this relationship going and is she okay with spending less time together? That kind of thing. Really get her thinking and going here. It's very possible she might not respond to that but you should try that first. Then evaluate your options later.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.