The issue of whether someone who is not currently holding office can be impeached by the House and convicted by the Senate is an open question. The House impeached Judge Belknap after he resigned but the Senate acuitted (largely because there were enough Senators who felt that they had no jurisdiction over an official after he left office so as to prevent a 2/3 majority for conviction). The Belknap case involved a situation where, it was alleged, the resignation was for the purpose of "evading" impeachment. Its not clear what precedntial value it has in a situation where an office holder serves out their term.
In any event, given the Nixon precedent, there is no chance the House/Senate will pursue chimpy or cheney after they leave office.
Finally, on the question of executive privilege and former presidents, I refer you to Executive Order 12667 (
http://www.whitehouse.gov/news/releases/2001/11/20011101-12.html) which sets out the current policy regarding the invocation of executive privilege by incumbent presidents on behalf of former presidents.