There are two obstacles to answering that question. First, I'm not sure exactly what you mean by "blow up the us". Second, there have, fortunately, been no direct observations of the use of many nuclear bombs in a short time on cities, so there's some uncertainty about the net effect.
Still, I can give some rough estimates. A typical modern H-bomb has explosive power of 300,000 kilotons, about 20 times as much as the bombs used on Japan. Much larger bombs exist, but these are the main ones in the U.S. and Russian arsenals. Dropped over a city, a bomb like that can easily kill 500,000 people rapidly. 200 such bombs dropped over a range of cities would kill something like 100,000,000 people (out of about 300,000,000) and destroy almost all our infrastructure for energy (oil refineries, power grids,...) communications, transportation, manufacturing,... The resulting soot would probably cause cool dark weather, disrupting even what farming could be carried out without fuel, fertilizers, etc. Radiation sickness would be widespread. Probably most of the people not killed in the initial strike would soon die. My personal guess is that bands of hunter-gatherers and small-scale cultivators would somehow survive.
Just for calibration, the U.S. and Russia have between them some 18,000 nuclear warheads. Minor nuclear powers (U.K, France, China, India, Pakistan, Israel) typically have 100-300 warheads. Back in the heyday of the Cold War, the purpose of the extra bombs was often described as "bouncing the rubble". At major defense contractors, one would hear assessments of the effectiveness of their products in "megafu's"- millions of family units to be killed.
(published on 06/11/12)