I doubt the general public would approve of a budget for buying thousands and thousands of AMD video cards. I'm not so sure that would get past any budget hearing.
I'm sure the government has enough computers that they could pull a big enough hash rate to seriously cripple the system if the wanted.
By the time the government has to take this extreme of measures to control bitcoin, it will already be so huge that it will be uncontrollable. For this to happen, the government would have to allocate all of it's computing powers RIGHT NOW in order to take down bitcoin, but with the growth in terrahashes that is now being seen, soon it will be impossible instead of just extremely extremely unlikely