As a couple, try to figure out what she is getting from online. And if any way you could provide that. Internet cheating is still cheating. But she started going there for a reason. If it is a way far out reason you might want to consider whether you can trust her or not. If you're invested in the relationship and she is too, you might want to try a couple of sessions of Couples Therapy. Good luck :)
If a person is going to cheat then nothing you say or do will have a lasting effect. They won't cheat if they love you, and if they don't love you then you need to move on anyway. Jealously will never bring you anything but heartache. Time to find someone who does care.
It's difficult to cheat through a screen... in the "physical sense," anyway. If she hasn't actually "been with" the other person, well... technically, she hasn't cheated... eh... not really. There's a grey area when defining "cheating" in a this type of situation. Often, emotional betrayal (as it is in your relationship) can be/feel worse than physical betrayal. You most certainly DON'T have to "let it slide," you have the option of ending the relationship, or trying to work things out with her; the option of doing whatever you feel will make you happy. I can tell you this much: if you feel that you can no longer trust her, it'd likely be best to end the relationship. Trust is essential in any relationship, and once that trust is lost, it's very hard to re-establish. If you can't trust her, you'll both be very unhappy trying to stay together.