First, there's no way McCain will ever get nominated. The base of the party, who nominates the candidate, is too conservative. And Buchanan is an albatross around the GOP's neck - the guy still supports isolationism, for God's sake. He's never going to win anything in the 21st century because he's a 19th century Republican.
And that gets me to my overarching point. Conservatives have taken the party back from moderates in the past, and they are able - and hopefully willing - to do it again. It happened in 1916, when Taft (the conservative) kept TR (the progressive) from dominating the party. It happened again in 1964, when Goldwater won the nomination and the roots of modern conservatism were born. And if it happens that the conservative base of the GOP needs to clean house, they'll do it again. If Bush (who is a far cry from conservative on domestic issues) keeps screwing up on domestic policy, especially immigration, the conservative base of the GOP is going to react by supporting a strong conservative. That's how we keep the party conservative - by working from the inside.
The solution you and PR throw around - abandoning the GOP and voting 3rd party - will never work. Name one 3rd party officeholder in the federal government (hint: there are none). Heck, name one elected at the state level of any state! There are no viable 3rd party Presidential candidates. So if you want to work to actually elect someone, keep your vote and your efforts within the party.