Question, basically with registration and user profiles

I am using Drupal 6.19 with Apache 2.2.15, Mysql 5.1.46 on a Slackware 13.1 64 bit system. I want the registration to be quick and easy so at this point the only thing that gets asked is username, password and email address. I know you can alter things using hooks in template.php in the current theme. What I want to know is is it possible after registration for the first login ONLY to display a page stating that you should fill out your user profile, and how I would go about doing this, pointing me in right direction is greatly appreciated.

I started creating my user profiles with Advanced Profile Kit following the instructions here http://drupal.org/node/517182 (only added the modules listed on this page and any module requirements) and importing the data as the instructions here http://drupal.org/node/579468 but have not set access control yet I am managing the fields so I can customize what gets asked. What I want is one set of questions get asked if a user is male and another set if the user is female but there is some questions that should be asked to both male and female. I not sure how about doing this I thought about adding the gender question to the registration but not sure that is the best way to go, would I be doing this with template.php or should I be looking into jquery?

Last question. I have a section that asks country and I have all the countries in a selection list, I have also done the same for state/province but only listed all the states and provinces for USA and Canada what I want to do is if the user selects USA or Canada in the country to display the selection list if they choose anything else use a text box so they can enter it. If I am correct that has to be jquery because with php I would have to click a submit button before it was know what answers I provided?