To get back to the OP, of course the Republican Party isn't done for. We live in a two-party country, and it will remain that way for a very long time.
That said, the GOP is in big trouble. Essentially, the Republican coalition begun by Goldwater, raised on Nixon, and matured by Reagan, has been taken to its logical conclusion (i.e. complete and utter ruin brought about by its own hubris) by Bush.
Here's a great article by George Packer about this very thing. Although, as always, I don't agree with everything Packer wrote (he almost completely elides the historically monumental corruption of the Bush era GOP), he does provide a very thorough examination of the evolution of the conservative movement in the United States over the last 45 years.