You aren't going to see that. Bush need not be spectacular at all. All he has to do is show up and appear more comfortable, answer questions, and not ramble.
The bar for him is very low tonight. The word improvement will be hard for the press to keep out of the stories and that's all that has to happen.
I think it's very possible if he does even competently tonight he'll just forgo the final debate. If domestic issues are mentioned tonight he can accurately say they were already covered. Arizona is the site of the final debate and they won't care. There may also be a good excuse that can be manufactured -- perhaps the Iraq offensive.
The point being, if he's substantially ahead and the clock is running out, there will be no point in putting the ball in the air.
As a point of reference, Mondale "beat" Reagan by 19 points in the Gallup poll concerning their first debate. Reagan beat Mondale by only 3 points in their second debate which was remembered as his great comeback. He went on to win an electoral landslide.
I wish he would. Three debates is nothing more than a gift to the media's bottom line. Blow off the third debate and let the Dems spend the rest of the campaign harping on that while Dubya harps on what a disaster it would be to the country if we get a liberal Mass Senator for President.