Biden is next in line. At this point there is no plausible legal theory for his removal.
Next would be Pelosi. No reason for her removal aside from FReeper pipe dreams. Pelosi can be removed as Speaker, or removed from the House, only by a majority vote of the House itself.
Next in line is Robert Byrd, the President Pro Tem of the Senate, who is pretty clearly physically unable to serve. In the extremely unlikely event that Obama is impeached, Biden is impeached, and Pelosi is ousted, that would give the Senate plenty of time to see this coming and elect another President Pro Tem.
If all of those pieces somehow fall into place -- the president, VP, speaker, and president pro tem all removed from office before any replacement can be chosen for any of those offices -- then the order of succession goes through the Cabinet in order of when the department was created. First in line is State and Hillary Clinton.
A more plausible scenario for President Clinton is that Obama is impeached, and Biden chooses Hillary as his VP. "More plausible" is a relative term, of course.
I don’t think anyone cares about Biden, Pelosi or Hillary in terms of removal. They are Americans and NBC.
No matter that we couldn’t diagree more on issues but they are not the crux of the issue and won’t be after this is resolved.