So would it really be possible to buy bitcoin at MtGox at $259 and sell for $635 at BitStamp at 18th February 2014? Because those are the bitcoin exchange prices at that moment?
In short: At this current time, you can buy BTC but you can not withdraw your BTC from Mt. Gox due to immaturity of their infrastructure and modified Bitcoin code. Time will tell if they survive this. So no, it is not possible to buy from GOX and then sell those at Bitstamp since Gox will not allow this until further notice expected before thursday.
At the beginning they will do so at a moderated pace and with new daily/monthly limits in place to prevent any problems with the new system and to take into account current market conditions.
'Withdraw' means moving your BTC or money out of the account to another account. It doesn't say it but it won't let you if you try. Google for Mt.Gox and you will find a lot of news about the current problem.