The fact that Pelosi is Speaker and, thus, third in line to be president is irrelevent. If impeachment procedings were to take place against both Bush and Cheney they would be in separate trials. If Cheney were to be impeached, Bush would get to appoint a new vice president. If Bush were to be impeached, Cheney would become president and get to appoint a new vice president. The only way Pelosi gets to be president is if both Bush and Cheney die at the same time.

The House issues Articles of Impeachment, but the trial takes place in the Senate. All that's needed to issue Articles of Impeachment is a simple majority in the House, but conviction requires a 2/3 majority in the Senate. It's pretty unlikely that the Dems can come up with the additional votes needed to convict in a Senate where they only have a one vote majority.

Anyway, if Bush were to be impeached, Cheney would become president which most Dems see as even worse than Bush. The only way to avoid that taking place is to impeach Cheney first. The impeachment process takes about a year. So, by the time they were to spend a year getting rid of Cheney and another year getting rid of Bush, Obama would be putting the finishing touches on his innaugural address.