But you can take over the network in a way that will crush it and make it totally unusable.
Not with any known supercomputer in the world, or even multiple of them.
Even the Tianhe-1 supercomputer would be pretty crappy for forging, at 7168 S1070 tesla gpu's, it comes to around 1.1 terahash per second, less than 1/8th or 12.5% of the total network power of Bitcoin.
And anything faster doesn't even exist at the moment.
This is discussed multiple times in other threads. Attack or compromise pools, buy all ATI cards you can buy, write disruptive Bitcoin client and the network will experience DoS.
And NVidia cards are crap. They are handicaped both in games and bitcoin mining.Nvidia are selling rebadged cards for multiple generations in the same time ATI are making new generations of powerfull and cheap cards. In past decade the opposite was true.