After the connection drops in the game, but before you reboot, can you connect to the Internet such as surfing to amazon.com?
Do you have an old nForce mobo? Check on nvidia's website for new bios update for your version of nForce.
Also, does this happen in every online game, or just some? For instance, Battlefield 2 released a patch that really increased server crashes and had some other netcode problems. Try to get the current patches for every game that has this problem.
Is it always around the same time that you lose connection? And are you really losing your connection or does the system hang? My point is that maybe after some intense gaming you are just overheating your graphics card and/or the system.