How long until you see improvement in search placement?
If your site has already been on the Web for some time, and is already listed in all of the major directories, then you will probably begin to see improvements in your search engine rankings within 4-6 weeks.
If your site is not listed in the major directories, then adding it to the directories will have a significant positive impact. If you pay for expedited review of your site at Yahoo, LookSmart, Business.com and others, than you will see improvement the next time Google, Yahoo, Bing and others update their spider-driven indices. You need to be prepared to wait for your site to be added to the Open Directory. Some sites have waited over a year to be added.
A total rebuild of your website will result in an improvement in your search engine rankings relatively quickly. You should continue to see improvements for 6-9 months, as the changes to your site are indexed throughout the databases that drive the major search engines.