We could impeach for stupidity. All that is required is the political will.
Impeachment is a POLITICAL process. It is Not a LEGAL/Judicial process.
If we had intended impeachment to be a legal process that met the standards of legal proof, we would have given the power to the courts. We did not. We gave it to Congress to ensure it was as direct an expression of our POLITICAL will possible in our Federal design. Unlike the laws passed by Congress, there is NO appeal on impeachment. No legalisms or "technicality" can trump our will as expressed by the men and women we elect to the U.S. Congress to represent us.
The definition of "impeachable" is 100% up to each Member of Congress to judge for themselves. They can look to authority. They can look into their own hearts. They can poll their constitutions. They are bound only by their oath to "support and defend."
The Hamdan ruling was a declaration that Bush and Cheney's arbitrary declarations that Gitmo was a Geneva-free zone constituted three years of War Crimes. There is no "unringing the bell" on War Crimes. Such crimes are subject to the penalty of death for a reason -- to motivate those with the power to commitment such crimes to steer clear of "the line." And "I thought the line was somewhere else" is not a defense.
Even if Bush and Cheney hadn't committed war crimes that have ALREADY been ajudicated by SCOTUS, the fact that 58% of Americans "Personally want GWB's presidency to be over" (Newsweek, January 27, 2007) is enough to impeach if the political will can be mustered to do so. (And if impeachment were "on the table" that number would undoubtedly jump at least 5 points overnight). A case could certainly be made that the betrayal of public trust reflected in such numbers is a "high crime" or "misdemeanour" against our Constitutional democracy.
There are FAR graver crimes, but the point is that there is no external, objective, binding authority that defines "impeachable."
Further discussion of what impeachment is, and is not, here:
http://journals.democraticunderground.com/pat_k/20