I'm sorry to hear about what you're going through, Steve.
From a purely distant perspective, I agree with baffled. It sounds like two issues here: her time spent online is out of hand and the fact that she thought it was okay or exciting to have virtual sex with someone outside of her marriage. Both are troubling, but the latter really is an indication that there's more wrong than just an addiction to the Internet.
Couples counseling is a terrific idea, but she also needs to be in individual therapy. From someone who's been in a sort of similar situation, when you do things that you know are destructive and hurtful, it's usually because you feel like there's something missing inside yourself, some void you need to fill. As beautiful and wonderful as you think she is, there must be a part of her that doesn't see what you do. If I'm guessing your ages correctly, she's probably near 40? That's a hard time for a lot of women - you have to start thinking about menopause, whether or not you're where you want to be in life, what kinds of things you have regrets about. It's hard to put it in words very well, but it could be that part of what she was looking for online was some "other" life - one that's exciting and dangerous and where she could pretend to be someone that she can't be with you.
In a lot of ways, it's going to be harder on her to get past this than it will be for you. She's the one who did this thing, who got caught and who hurt you. When she does come to her senses, she's probably going to have a lot of guilt, and I really would encourage you to keep going to therapy for a while.
As far as how to keep her from doing this again...she has to get to the underlying reason she did what she did, and again, there are two separate issues. I think there would be ways to limit how long she's online, but she's an adult - she has to take responsibility for her own addiction and she's going to have to deal with it. You can't monitor her forever, and you shouldn't. As far as the virtual sex and chat goes - again, she needs to figure out and fix whatever it was in her that drove her there. That's more difficult to fix and will take longer - it'll also be the part that's harder for you to get past.
Good luck - I really hope things work out for you and that you and your wife will work through this. It sounds like you're doing the right things right now. Your wife is a lucky woman.