1. Register Democratic and vote for Obama? He seems like the least-bad leading candidate. (Some evidence. The linked post takes this as evidence of effectiveness; I'm more impressed by the choice of what to be effective about.)
2. Register Republican and vote for Ron Paul? There will be a Republican Party for the foreseeable future, that even wins some elections, so its future character matters a lot. Paul, for all his, ah, imperfections, is the only candidate to really repudiate the Bush administration (besides repudiations like it's too liberal or not warlike enough); a good enough showing might make some marginal difference.
3. Register Republican and strategically vote for Romney? I suppose keeping McCain out of the general election, which he might win with the press fluffing him, would make a bigger difference than influencing the Democratic choice. Downside: this is dishonest.