I see potential for nintendoforums.net and gamecubetalk.com, but the problem is that people who run forums don't necessarily have a lot of money to spend on a domain name. Maybe a couple hundred if they're really serious about developing.
The great thing about nintendoforums.net is that it allows for new systems and old systems - not just the gamecube system. And the .net extension is actually better because it's more accurate. If I were you, I'd promote this one in every free place possible.