Would Speaker Pelosi be president if Bush-Cheney impeached?

Since political office holders are (should the need arise) impeached individually the situation isn't likely to ever arise. Articles of Impeachment are drawn up against one individual and each would have seperate hearings and votes in the Senate.

If the VP was impeached a new VP would be appointed by the President. If a President's impeachment happened frst then the VP would become President and would appoint a new VP.

There is no provision for a "double impeachment" where both would be impeached at the exact same moment.

If the President were impeached and removed then the VP would become President. To put a new VP in place the 25th amendment would have to be followed.

If the intent of Congress was to also impeach and remove the VP, it would be unlikely that he would be able to nominate and get a new VP approved by both houses in Congress before his removal.

Without a VP and the President being removed from office then the standard succession procedure would take place and the Speaker of the House is next in line after the VP.

I would submit that it would be much more likely that the house and Senate would approve the nomination of a new VP than it would be to complete the impeachment.

An impeachment requires a 2/3rds vote of the Senate while the nomination approval only requires a simple majority in each house.

Whichever party controlled the Presidency would be unlikely to allow the impeachment of both the President and VP without a confirmed successor of their choosing.

The question wasn't how likely it was to occur.

If by some chance there were enough votes to impeach and convict both the President and VP, I don't think it is likely there would be enough votes to allow either to put in their own hand picked successor without some kind of a deal with Congress. (I started to put that in my answer but realized it wasn't the question.)

In order to impeach both it would mean there must be some very real constitutional crisis that would make it unlikely that they would be trusted to decide their own successor.