Dear Stalked,
OK, I'll make this simple. Run, don't walk away from this situation. Is it likely to get dangerous? Maybe, maybe not, but you don't want to stick around to find out. Somehow you've become caught in your own personal Bermuda Triangle, complete with the lying and cheating (but concerned) guy and a just-slightly off girl who doesn't mind reading people's mail, pretending to be someone else and dialing up strangers. Hopefully, you don't see yourself fitting into this picture. By the way, which chat rooms are you hanging out in? I might visit to spice up a slow weekend sometime.
I'm not sure what else you can do other than let things fizzle out here and let this girl run out of energy. While getting a restraining order might be a little severe this early, I'd be prepared to do it if things continue, and I'd definitely threaten to do it the next time you talk to her. In fact, you might want to pick up the phone and let her know that you're going to provide the police with her name and address next time she calls. By all means, stop talking to this guy, no matter what the circumstances. Other than that, I don't think you can or should do much – Sid and Nancy will get bored and find someone new to fight in front of. It also might not hurt to stay out of your usual chat room(s) for a little while – if you're ready to get on with your life, you might want to start by changing your scenery, at least for the time being. In hindsight, I'm guessing that confronting her once you found out who she was gave her the drama and attention she wanted.
Be careful out there in cyberspace. It concerns me that you spent weeks talking to a "guy" that you thought was suspicious but yet you needed your creepy ex to confirm that something was amiss here. If you're giving out a lot of personal information about what you're up to, and you don't even know who you're talking to, you might just be inviting some unsavory characters into your life. The net can be a lot of fun, but it can also be a haven for weirdos – the anonymity that the net offers means that you have to be extra careful in knowing who you're dealing with.
Sincerely,
Mike