Nixon was not impeached. The appropriate House committee voted to do so but he resigned before being impeached.
The only president impeached in the 20th century is Bill Clinton, darling of the Dem party.
Nixon was impeached by the House, but not convicted by the Senate. He would have been, but he chose to resign instead. The telling moment was when Barry Goldwater said he would vote to convict. After that, Nixon knew his fate was sealed.
Slick was impeached by the House, but not convicted by the Senate. As Trent Lott said "if we start impeaching people for having sex with people not their wives, there won't be many of us left." He voted to convict, though.