Impeachment would be a serious mistake unless that you believe all Bush deserves is a slap on the wrist for the crimes he has committed. Make no mistake: impeachment would be a slap on the wrist for the nature of the the Bush crimes.
I agree with Vincent Bugliosi:
If Bush were impeached, convicted in the Senate, and removed from office, he'd still be a free man, still be able to wake up in the morning with his cup of coffee and freshly squeezed orange juice and read the morning paper, still travel widely and lead a life of privilege, still belong to his country club and get standing ovations whenever he chose to speak to the Republican faithful. This, for being responsible for over 100,000 horrible deaths?* For anyone interested in true justice, impeachment alone would be a joke for what Bush did.
To those who claim that we could also go after him for murder and/or war crimes, I think that's highly unlikely. First, if as is likely, he gets acquitted in the Senate, he'll be able to claim vindication making it harder to bring any additional charges against him after he leaves office. Second, in the unlikely event there were to be a conviction by the Senate, he could then claim he had already been punished and should not be subject to additional punishment for his crimes.
Keep in mind that punishing a former President for war crimes and/or (mass) murder is unprecedented and it would take an inordinate amount of education and convincing to get the American people behind it at or at least accepting of the idea. Subjecting Bush to the impeachment process first, regardless of the outcome, would make it all that much harder.
I would prefer to see Kucininch and Wexler and a couple of Senators go on the record now in support of criminal charges against Bush and others in his administration once they leave office and to make it clear that committing any more crimes such as aggressive war against Iran will add to the charges under consideration. Put that gang of thugs on notice that they won't get away with murder!