Most of the stuff that I have read about espalier suggests that peaches should be trained into a fan shape rather than a traditional horizontal cordon espalier. That form more readily allows you to prune annually for renewal wood to fruit the following year.
Earlier this year, in the Pictures section, I included a picture of a large peach fan-trained espalier that I saw in France last summer. It is the third picture down in this post:
http://growingfruit.org/t/espalier-trees-at-le-potager-du-roi-in-versailles/4064