It is fairly obvious that the only reason the Democrats are holding back on impeachment is they are afraid it will hurt their political chances in 2008, whether they remove Bush or not.
The Democrats want Bush and Cheney in place so 2008 will be a referendum on their corrupt, destructive government. If they are removed, they will be history, and for Americans, history is anything that isn't in the current news cycle, and only a handful of eggheads care about history.
They also mistakenly believe the rest of us are so stupid we can't tell the difference between the politically motivated impeachment of Bill Clinton and trying to remove the most dangerous, reckless, and corrupt president in our history.
However, if they impeached Bush & Cheney and they was NOT removed, they would not only still be in place, but the media would be forced to cover the hearings and a lot of the public would be hearing about their impeachable offenses. That would make the anti-republican tsunami even greater in '08.
But it would also make the Bush & Cheney offenses so obvious, enough republicans would vote to impeach,just to get the albatross off their neck, and they would be removed.
While I would prefer to see both removed, and sent to prison here or sent to the Hague, impeaching just Cheney could meet the needs of our constitutional system for accountability, and still leave an emasculated Bush in place as the political punching bag they want. With Cheney gone, half of Bush's brain would be gone.
And impeaching just Cheney is exactly what Dennis Kucinich proposed in his bill.
The objection to this is it may give Bush a chance to give one of the GOP candidates for president some prestige by appointing them as VP, and possibly catapult some seeming moderate to the GOP nomination who might otherwise not have made it through the gauntlet of American Taliban voters (if Bush appointed a far righty, that would be the best of all possible worlds since the guy would have the same pedestal, but be doomed in the general election no matter what). A 'moderate' VP would have a real problem as long as Bush was in office though--he would have to defend the corruption, destruction, and failure of his boss, so his appointment would only be a gift if Bush resigned or was impeached himself.
I doubt that Bush would do something as selfless as resign just to help the party.