Search Engine Friendly Design - Web Site Architecture and Link Structure
In order for the engines to be able to easily get around your site and index every page you will ideally have a text based navigation structure. Drop down menus, image maps and frameset menus are difficult, and in some cases impossible, for the search engine spiders to follow. So try to make it easy on them by either using a completely text based navigation system or providing text links throughout each page's copy.
If possible, all of the site's pages should reside in the root directory. Where there are too many pages for this to be feasible, try to keep the site architecture as simple and as close to the ground level as possible. Avoid having a url that looks like www.mysite.com/directories/level1/level2/page1.htm. The further from the root the page is, the less likely it is to be crawled.
The link structure should allow the user and spider to get to any page from any page. In very large sites this will translate to any directory main page from any page. The user should never be more than 1 or 2 clicks away from where he or she wants to go and the path to get there should be so clear that even a blind person can't miss it.
Since you've already researched and divided up your keywords for each page give each page the main keywords as a file name. If it's 2 words separate them with a dash. Same goes for directory names. So instead of having www.mysite.com/directory/page1.htm make it www.mysite.com/widgets/blue-widgets.htm. Your files and directory names will not make or break your listings but it can't hurt and every little bit extra may help (this is also true with domain names).
Do not obsess over it and register domains or make up directories and very long file names just to use keywords as that can make it very difficult for people to remember the url and increase the chances of it being mis-typed. As in everything to do with your site, your primary goal is not to be found but to sell. You have to balance every aspect of your site's design between what is good for search engine optimisation and what your target audience needs.
Search Engine Themes and Page Rank
NEXT Step: Search Engine Friendly Frame Sites