Iran won't be able to build a boosted uranium fission bomb, no matter how many centrifuges they have. The best they could hope for would be a bomb the size of Little Boy or Fat Man, both of which had yields under 20 kilotons. It takes years of testing and experimentation (which would be immediately known by us, Russia and China) to build a simple uranium fission bomb - and years more testing and research to progress to further designs. You can't just build an H-bomb.
Heh, so, you think Iran is limited merely by its own technology? I seriously doubt that. And you forget how long the Iranian nuclear program has been active.
Alright then, so, Iran has enough nuclear fuel at the moment to make 7 implosion style nuclear warheads (that of which you just described)
If they launched just 5 and detonated them 4 miles above the surfaces at Tel Aviv, Jerusalem, Haifa, Rishon LeZion, and Ashdod (Israel's 5 largest cities), you would see nearly 300,000 people die at once. It took 9 years for that many to die from the initial explosions, radiation burns and exposure in Hiroshima and Nagasaki. Given how small Israel is, the death toll would be magnified.