content based on location and User's IP address

I need to create a Drupal site for a client with content based on location as seen here: http://www.examiner.com/anchorage (click "change city" button next to logo on top of the page). Anyone could tell me how to get started and what modules I should install? It would also be interesting to have an option to automatically detect the user's IP address to show content based on his location when he first visit the site.

I search on the forum already but didn't find answers to my questions.

Thank you.