Since World War II, the specter of impeachment has haunted two Presidents, over widely different circumstances.
Richard M. Nixon stood on the throes of impeachment due to his interference into the investigation of a reprehensible political operation.
William J. Clinton was impeached and tried due to his prevarication over a personal indiscretion.
George W. Bush's situation is drastically different from both those men's. His political problems don't stem from underhanded electioneering or an inability to keep his trousers fastened; they stem from the fundamental, zealously advanced actions and agendas of his entire administration, and if he is impeached, it won't be a distraction or a sideshow; it will be a referendum on his stewardship of the office of President of the United States.
It's also useful to look at the various Vice Presidents involved.
Spiro Agnew was rather a peripheral player in the Nixon White House, and ended up resigning practically on a technicality (groceries as bribery?); his successor, Gerald Ford, was something of an outsider, so he didn't suffer as much from the taint of Watergate.
Al Gore was very much a political ally of Bill Clinton, but Clinton's impeachment wasn't predicated on his policy or politics, but on his personal foibles, and not even Ken Starr would have been able to propose that Al Gore had been "ahem" personally involved in the Lewinsky fiasco.
Dick Cheney, on the other hand, is intricately linked to the policies of the Bush II administration; he's seen as the eminence grise behind the facade of His Royal Imperious Shrubliness George II, and if Bush is removed by impeachment, Cheney will be sworn in - facing a Congress which will have already given him an unprecedented vote of no confidence.
Face it: Cheney's in it up to his neck, and if George II goes down, Tricky Dick II won't have the political clout to call for a time of national healing.
In fact, the end game might not result in impeachment after all. If the Congress flips in '06, and the Downing Street Minutes or something even bigger and more damning dominate the headlines, then the result may be a plea-bargain much like Nixon's.
Only the price demanded by the prosecutors may be two resignations, not one.