The two questions are synonymous; if something is "necessary", isn't it also "needed"? Because there were two questions instead of one, I'll answer as if the first question is asking "If we were to rescind executive privilege, would a Constitutional amendment be required?" and as if the second were asking "Should executive privilege be rescinded?"
Do we need an amendment to revoke "executive privilege"? During the Jefferson administration, the Marshall Court ruled that executive privilege was necessary, but wasn't absolute, so the Judiciary could mediate in rare cases to determine whether claims to privilege were legitimate. However, that ruling has never been used, because one side or the other has ultimately blinked before it has come to a court-mediated showdown.
(more)To completely overrule executive privilege, the Supreme Court would have to come to the conclusion that the Constitution does not protect the separation of powers and does not intend the three branches of government to be co-equal. If we ever get a Supreme Court willing to make such a ruling, they're the type willing to amend by fiat, so exective power will be the least of our worries, considering the only practical checks on the Supreme Court are resignation or death followed by Presidential appointment and Senate advice/consent.
A Supreme Court willing to overrule the separation of powers is probably also willing to make itself a
de facto Congress and President, too. So, we'd better hope that we never get such an unwise, power-hungry Supreme Court. Instead, if we the people decide we must rescind the privilege that keeps the Executive and Legislative branches separate and in balance, we'd better do so through an amendment that makes clear which other separation of powers remain intact.
If yes to an amendment- is it neccesary? No-- in fact, it would be quite unwise, and would throw the fundamental understanding of Constitutional law completely out of whack. Why? Congress is supposed to have as much Constitutional authority over the President as the President has over Congress, as specifically outlined through the system of checks and balances. There are large swaths of the Executive branch over which Congress has "oversight" because Congress enacted those portions of the bureaucracy into existence, and therefore has the power to revoke or reorganize them, to differing degrees. However, because the President and Vice President are Constitution-level officers on par with Congress, Congress has no power over them that isn't directly written into the Constitution; and similarly, the President has no power over Congress except those granted by the Constitution.
If Congress could investigate the President willy-nilly without substantiated impeachment-level allegations, then the President could also order the DOJ to investigate Congress and the Supreme Court on a whim, and force them to reveal everything done in secret under threat of perjury charges; or, just harrass them into compliance by having them tailed by the FBI and arrested for jaywalking, loitering, failing to yield right-of-way, etc. So, the baseline is that without some substantial allegation of specific criminal behavior, the Executive branch can't investigate Constituionally-created officials of the other branches, and vice versa. But like any other investigation, once an investigation starts, it's possible to explore evidence of other crimes discovered along the way, including cover-up crimes like Clinton's perjury in the Whitewater-Jones-Lewinski affairs.
Due to the low ratings of the current President and the even lower ratings of Congress, I'm predicting Clinton will retake the White House and Republicans will retake Congress by 2008 or 2010. Revoke executive privilege, and Clinton II will be served papers shortly after her first steps back into the White House concerning her involvement with Whitewater, Travelgate, her book deals, her campaign aberrations, etc., and it will just snowball into a major mess as every tiny mis-step gets rolled into impeachment charges. She's no Teflon Bill-- something's going to stick. Or, they might use the Kucinich scheme and try to impeach Clinton II's Veep, so that a Republican Speaker of the House will ascend to the Presidency.
Such dragnet politics never do the country any good. Neither does eroding the Constitutional balance and separation of powers until the President is nothing more than a second Speaker of the House, without any power.
The Founders were far more worried about a runaway Parliament than they were about a runaway limited Executive. Although the final draft of the Declaration blamed the Crown for mistreatment of coloniists, if you study the specific allegations, they were all acts of a democratically-elected British Parliament, and his precursors to the Declaration didn't mind calling out the Parliament directly. In
A Summary View of the Rights of British America, a draft of a letter to King George begging him to reign in Parliament, Jefferson blames on Parliament many of the same grievances he will later blame on the King in the Declaration:
QUOTE
That to heighten still the idea of parliamentary justice, and to shew with what moderation they are like to exercise power, where themselves are to feel no part of its weight, we take leave to mention to his majesty certain other acts of British parliament, by which they would prohibit us from manufacturing for our own use the articles we raise on our own lands with our own labour. By an act passed... [lists 11 oppressive acts of Parliament]
In a defense of the Declaration, Jefferson notes that even though the Crown hadn't overturned an act of Parliament in about 3 centuries (longer than the U.S. has even existed to date), Jefferson thought it legitimate to blame the King for acts of Parliament because the power to veto acts of parliament still theoretically existed. (
sources) In reality, it was just easier to get colonists, who considered themselves Englishmen, to rally against one unpopular monarch than the whole of the British people represented by Parliament.