A few days ago I read a article that said that the WTC attack was the equivalent of approximately .25 KT. Of course, that was spread out over, what, an hour or so? A nuke would deliver its damage over the course of a split second, rather than over the amount of time it took for the jet fuel to burn.
To put that in practical terms, I can tap you lightly on the cheek a hundred times in a row, and you'd barely feel it. But if I delivered the same amount of energy in one blow, you'd be sent reeling.
OK, so now that we know about how much energy was released on 9/11, and know what level of damage it caused -- and know that the damage would have been quite a bit more severe if the .25 KT were delivered at once rather than spread out over a protracted period of time, we've got at least a rough benchmark of sorts.
According to the FAS site mentioned in post # 109 in this thread, the ex-soviet suitcase nukes have a yield of 10 KT. So, a "suitcase" would have about 40 times the yield of the WTC attack, except, of course, it would be delivered instantly rather than meted out over an hour or so.
Not nearly a "city buster", but nothing to sneeze at either.
Suitcase nukes have yields up to around 1 kiloton (sometimes much less, the smallest US nuclear weapons had yields of about 0.02 kilotons).
1 kt could cause a hell of a mess, but would nowhere near take out a whole city, or even a large chunk of one. Major destruction would be limited to a few city blocks, more than we saw in the WTC attacks, but not immensely so. Of course, there would be no time to evacuate as was the case for WTC before the buildings came down, so the number of fatalities could be a lot larger if a heavily populated area were chosen.
The area of more or less complete destruction would extend about 0.35 miles (1800 feet) from the point of a 1kt explosion. The amount of damage beyond that area would drop off rapidly, and buildings and people as little as one or two miles away would be relatively safe.
It wouldn't be at all like the results of the 20+ megaton (20,000 times as powerful) thermonuclear H-bombs that superpowers have in their arsenal. This is what most people think of when they hear "nukes", but those are the way upper end. And an H-bomb is *way* beyond the technical abilities of terrorists, or even Iraq and other countries.
As for radiation from a 1kt bomb, there would be time to evacuate most of the people at risk from the downwind plume. And the lingering radiation at ground zero would be dangerous for a week or two, but the radiation from the more powerful 15kt bomb that was dropped on Hiroshima was down to relatively safe levels (5 times normal background radiation) eight weeks after the explosion, and Hiroshima has been continuously inhabited ever since.
Total, WTC or Hiroshima-style destruction, and 75-100% casualties, perhaps a few blocks up to 1/2 mile.
Substantial (50-75%) building losses due to fire & structural damage (i.e., quite a few buildings may have to be condemned, torn down and rebuilt), and 50-75% casualties, perhaps a 1/2-1 mile radius.
Considerable (25-50%) building losses due to fire & structural damage (i.e., extensive repairs required to most buildings), and 25-50% casualties, perhaps a 1 - 2 mile radius.
Most buildings survive with only minor damage (windows), 5-20% casualties limited mostly to those caught out in the open, perhaps a 2-3 mile radius.
Minor structural damage and small percentage of casualties (many cases of eyesight damage or loss, for example) in a 3-10 mile radius.
Some people get sick, and a small but elevated percentage eventually die due to radiation sickness, cancer, etc., downwind of ground zero.
Remember that quite a few of the buildings nearby that are not demolished by the blast will be finished off by fire. No city has a sufficiently large fire department to respond to all the fires that will be set off by a nuclear detonation anywhere within its borders. This can include wood frame houses up to a mile or more from ground zero, although the fact that with terrorists it would almost surely be a ground burst works to limit this effect.