I think McCain would either pick longtime ally Lindsay Graham or maybe Mark Sanford. He'd easily beat Gore, though a lot of the religious right would go to Alan Keyes or Bob Smith (Constitution/US Taxpayers), or Pat Buchanan (Reform). Nader's vote might be smaller, though, with McCain's reputation for reform.
McCain would keep the surplus for longer, thus reducing the debt. Unlike Bush, he'd be willing to investigate certain actions taken by Clinton. He'd be willing to go into Iraq sooner (or North Korea). I'm not sure how he'd handle the intel on 9/11. He'd take a tougher line on China in the EP-3 incident. After that, who knows? The only decision I think he'd do, if he's in the WH in 2008 is to get tougher on Russia after they intervene in the South Ossetia War...which could become WWIII. (I am working on a McCain TL, in which at least two prominent Republicans leave the party and get elected to office on third party tickets as a result...and McCain is the first Republican to face a certain problem in about 140 years...)