Hi guys

Ok im totally new so dont flame me if this is a stupid question.

I joined 2 different trading platforms, mtgox and bitstamp. I saw the price per bitcoin on mtgox is £233 per coin ($389), where as to buy a bitcoin on bitstamp is $647

Well doesnt that mean i can buy a coin on mtgox and sell it on bitstamp for nearly twice as much?? Or am i missing something and they are not interchangable? so i cant send my coins from mtgox to bitstamp etc?