Well since GameSpy is out of the question I looked around a bit and found RakNet. Turns out it's also free for us indie devs as well. What's even better is that they've done most of the hard work for us. Included is the support, setup, and documentation for using Swig to wrap the C++ dll's up for use in .Net. So yay! Windows Networking support without having to roll everything from scratch!
On that note I have a small side project used as a testing bed for learning that piece, more on that later.
The main project is progressing, if slowly. Got tunnel shadows implemented with remarkably little work, and it looks pretty good too.