If the Golden Dawn actually takes power and starts with racist bullshit, I expect they'll be swiftly expelled from the EU.
Greece wants to leave the EU. The EU doesn't want to let them leave. If Greece leaves the EU (defaulting on their loans as they go, of course) they can engage in the kind of external devaluation the Euro won't allow. Then maybe they can export their way out of the recession. Complete disaster for Germany of course- no more easy access to uncompetitive Southern markets for you to impoverish.
I think you're mixing up EU and eurozone, which is not the same thing. To devalue the currency they need to leave the euro, which is an idea that has been bandied about multiple times over the last years. It would be a clusterfuck on its own; whether it'd be worse than the ongoing clusterfuck I'm not judging.
But leaving the euro doesn't automatically mean leaving the EU, too. It might be done in addition, but it's a whole other business. Though I'm pretty sure the GD would be all for it.