as in "I did not have sexual relations with that woman, Miss Lewinsky."

Saying something you believe to be true that turns out later to be wrong isn't a lie. If it were, then everyone would be a liar because we've all said things that later turned out to be untrue. That's hardly an impeachable offense.

If Bush received intelligence reports that there were WMD in Iraq, then he wasn't lying when he said they were there. He was repeating information told to him by the people with the responsibility for knowing such things.

If proof exists that the intelligence reports showed no WMD and he stated otherwise, then you'd have a case. However, given that the UNSCR 1441 (which passed unanimously, by the way) indicated that Iraq had WMD, then how is Bush lying if he also says the same thing?