If the fanatical Arab goal is not to defeat other nations militarily, but to send the world back to the dark ages, then a black-market purchased nuke detonated in NYC or Wash DC will do just fine.
While such an event would certainly disrupt things, I do believe the US could forge on without either DC or NYC, or both. However it would lead to dark age alright, in the Arab world, although for most Arabs, those that survived that is, the differences would be minimal.
Actually I think that a black market nuke detonating anywhere in this country will get the arab midlle east turned into a slag of glass. The geeks at the UN would literally drop a load in their pants the instant that word arrived that a nuke had flattened Washington D.C. also the folks over at the HAMAS front group CAIR would find themselves seriously running for their lives as would any campus douche bag that tried running on at the mouth about how evil the USA was & that it got what it had coming.