geographic search prototype for geotarget advertising system

I have a Joomla website that i want to extend with advertising based on geographic location. I need a prototype of the functionality before i can define a full project. From the website i want the geograpic location of a user based on his IP adress. If not possible based on IP or if the user wishes to do so he may give a different location. Using that location i want to search for an area that the location falls into. The area will in turn determine what advertisements are to be show to this user/visitor. You must tell me specifically how you intend to achieve each part of the functionality. Tools, services, libraries etc... so that on the basis of your suggestions we can define a complete specification to build a prototype for this functionality. 1) determine location. 1a) Based on IP adress, how is this done, how do you go from IP to longitude/latitude ? 1b) How can the user be allowed to enter a location whereby we can extract longitude/latitude accurately? (google?) 2) determine area 2a) Are there services/libraries available that allow a search based on latitude/longitude that return an area indication such as postal code for example. (google?) 2b) Assuming i must have my own database with area's defined by a series of points. Note there are no empty area's and there are no overlaps. How will you determine what area a point falls in using MySQL/PHP or other tools ? Again, you need to provide specific suggestions for me to consider and look at otherwise i cannot have a converation with you.