I guess you should start at the drupal site Customization and theming -- focus on 4.7 and PHPTemplate it is the default theme engine..

Learn from ichris.ws Theme Development Checklist is a good "how-to" to go from a cool HTML / CSS page look to a drupal theme

Drupal Tutorials has a good review with some tips... I find the more different stuff I read the better I understand something - after banging my head on the desk trying it for a while..

Some other resources are linked from Groups.Drupal-Theme Development

Related to the graphic design of a site is how to get it to do what you want ...here is a link to the drupal site discussion of Site configuration challenge: corporate brochure it gives lots of ideas on how to go about setting up a drupal site.

added ... from Nick Lewis: The Blog Learning How To Theme in Drupal, Starting at Square One

Just to keep up this post by Sam Raheb seems to be all over ... he built an excel spreadsheet that uses the "bluemarine" default drupal theme and allows you to modify it - then export your results for use on your site... weird way -- I may try it sometime -- I guess if you have a hammer everything is a nail -- he seems to be an excel macro wizard..