It's more difficult now because bitcoin has a large market cap, but for lower MC altcoins it would be very easy. Mt Gox isn't just about them dumping their coins, it's about panic sellers learning about gox moving their coins that causes the jitters
to shake up the price of bitcoin, of course very large money is needed, but there are many ways for traders to boost the market, one of which is creating spectacular news that can make bitcoin owners panic , by itself bitcoin owners will sell their coins crowded and it will cause a sharp drop in prices , in fact there are many ways that can affect the price of bitcoin in the market , but all of these methods are not easy to do.