The numbers "on paper" say we can, but when you figure in troop rotation, funding, and most of all the media management of yet "another" war (and I know, it's the SAME WAR, the ORIGINAL WAR, but it won't be played that way), well, I think we have to make sure Afghanistan and Iraq are completely set before we go after the next target. But I'm all for eventually taking out all these creeps.
If we strike into Iran, we'd need at least 200,000 ground troops. And we would probably have a nice buildup in Afghanistan- probably not for Iran-bound troops- but to pacify Afghanistan itself.
Teheran itself is in the north-central of Iran (slightly west of center), near the Caspian Sea. It's about 500 miles from the Afghan border, so invading troops would almost certainly come from Iraq. Another front could be opened from the Persian Gulf. Here's a good physical map of Iran:
This is probably all mental onanism. It's extremely unlikely that we will actually invade Iran. If there is a "grand plan" to take out these terrorist nations one-by-one, Syria would probably be next.
If we do invade either Syria or Iran, the stability of Pakistan and control of its nuclear weapons becomes a crucial consideration.