Leaving the euro will most likely be good for the euro. Like divorcing your big spending wife.
The question is really, if they default will they cause a bank like JP Morgan to fold?
Do a search on 'bank OTC exposures' and you'll get an idea of what's at risk here. Very likely we all go back to the stoneage for a while if JP Morgan fails....
Other than that Greece will do what Greece will do, doesn't even help to try worry about what's going to happen there, just roll with it as it unfolds.