But it could go either way. The US has as much to lose as it has to gain. Khamenei was/is a "status quo" guy. He vetoed some of the more reformist candidates but he also kept the hardliners in check. A new Supreme Leader could re-invigorate the conservative elements within Iran and turn turn back (or try) the creeping reforms and disillusionment with the Islamic system. He could also be more pro-active abroad. Khamenei has effectively been a steadying force and he's kept Ahmadinejad on a pretty short leash. If that were to change, the US and Israel could be facing a muuch different Iran. But then again, it next Supreme Leader could be Rasfanjani or one of his buddies, in which case the conservatives (Iranian that is) would have the carpet pulled out from under their feet.
I heard Iran wants to switch all transactions with oil to Euro, and other countries were expressing similar sentiments. Something like that would cause havoc with the USA economy.
They've already started doing it, just a little while ago. But I don't think that alone would have
too much of a negative effect on the dollar. The dollar is already doing badly, and other, more powerful countries (like China)are slowly starting to convert a portion of their foreign reserves into Euros. Venezuela was also planning to sell its oil in Euros, but I don't know what's going on with that.
As Gary Brecher said, if this does explode, genocide will probably go wth it, that or NBC 
Every once in a while, I just hope for a massive, epic, awesome war in the Middle East. What a sight that would be! Cause Iranians are tough and they're smart. No offence to Arabs, but they can't fight for **** and are too stupid to so much as fly a plane, much less build one. Persians on the other hand, are willy people, and Iran has enough manpower and conventional weapons to create a glorious war. Oh please God, let Bush be stupid enough to go pick a fight with Iran. Because they've been tooling up for it for 15 years.