The effect in Greece would be significant, imports would become expensive, but debt burden would be eased. Inflation would pick up steam. There will be immediate liquidity problems. In the Euro zone the effect in 6 months would hardly be noticeable. In global markets, other than in those closely tied to the Greek economy, the effect would be nearly undetectable.
The total amount of the Greek debt is a tiny drop in the international finance bucket. Greece could default on all debt tomorrow, never pay a drachma back, and the net effect globally would be virtually zero. It's too small an amount to be significant overall. US markets are reacting partly because of hedging against a market reaction, which is a bit ironic. Those who hold assets related to the Greek economy will rest uneasily unless they are fully hedged or insured, and those who see the market falling will panic as they always do. And of course the reaction will be over done, as it always is. For very short term traders this is, once the selling stops, a buying opportunity.
If Greece were to drop out of the Euro zone, it could actually help strengthen the Euro, by removing one of the albatrosses around its neck. It is possible that Greece would go back to the drachma, but remain in the Euro economic community. If I were to bet, I would bet on a last minute accommodation however..
Of much more impact will be the Fed's decision in the fall. The U.S. market has been trying to put in a top for months now, and this little fracas my help things along. Eventually it will fall, and of course the reaction to anticipation of tightening, which has already begun, will be overdone as always. The Fed will tighten, when they finally do, in a very gradual and measured way. Once people realize the sky is not falling, the market will recover some, but it will remain tepid for an extended period of many months if not several years. After all we have enjoyed six years of a continuous bull market following the 2007-2009 crisis. Now THAT was a REAL crisis!