I open this thread for anyone who is interesting in implementing Ajax on multiple parts of Zencart, the main goals are:

1. Implement ajax on current zencart pages/functions without breaking the default behaviors
2. Allow fall-back method when the browsers do not support javascript
3. Reduce load time by updating only part of the page (which is one of the main points of ajax anyway)

Anyhow, here is my approach:
1. When a page is loaded, check if it is loaded through ajax or not, if it is an ajax request the we use ob_start to store the output
2. We will then print out only the portion we need.
3. The javascript on the front-end will take the output and do the updates etc

Several obstacles so far:
1. zen_redirect function needs to be replaced
2. messageStack doesn't store enough information, it is needed to be replaced as well.