Best method to change part of page based on user selection.

Hi ppl,
I'm new to the .net environment (still stuck in the Classic ASP mentality) and was wondering if "update panel" (ajax) is the best (or even the right choice) to do what I want.
I'm hoping you guys could point me in the right direction. 
Basically, I have 2 parts in my page.  1st part allows user to select the TYPE of action (eg. add a food product, add an electronic or add a book).  Upon selection, I want the 2nd portion to show the required "form" (eg. product name, brand & expiry date for food.  book name, author, etc for book) so user can add the item.
The above is just an example of what I want to achieve, it's not my actual case. (in case you're wondering why I wanna do it) Should I do a "post" and re-render part 2 (when part 1 changes) or should I just load everything and use show/hide to ensure user uses the right "form" to add a record?
Any info or he ...