I think he will be seen as one of the 'also served's. His record was not remarkable, and many large mistakes were made. He presided over significant preventable problems, and didn't deal particularly well with the unavoidable ones.
He will not rank with the major presidential figures of the last century; FDR, Kennedy, Reagan, possibly Clinton.
Some former presidents can recover from a mediocre office by the works they do after; Jimmy Carter is a good example of this. However, I don't think it's likely W will end up in this category.
He will fade from mainstream public perception, and end up only as a mid-page answer in school tests.