To validate your HTML, there is a cool tool, called HTML tidy ... although this prog stand alone is kinda a pain to use, a easy version comes with HTML-kit.

HTML-kit is a pure code HTML editor, it is not WYSIWYG like DW. So its going to be slow to make pages ... if you really wan to make standard compliant XHTML for example, open you files in HTML-Kit and choose to validate your code, it will output very helpfull info, which line is not compliant which tag not properly ended etc ... super cool.

The best thing is both HTML-KIT and HTML Tidy are FREE!

Can check out HTML-kit here
http://www.chami.com/html-kit/

HTML Tidy can be found here
http://www.w3.org/People/Raggett/tidy/

An article on how to modify DW to produce valid XHTML
http://www.alistapart.com/stories/dreamweaver/
This article is a bit old ... is about DW4 have not tried doing it with DW MX, if you do let me know?

All da best!