Categories – display etc

I have searched the wp archives, but couldn’t find the required thing. My query is:

If there is a site with a single usual install of wp and there are 5 categories, and index.php shows everything relating to those 5 categories.

I want that category 1,2,3 be shown through index.php. And category 4,5 through newindex.php

This means that 1) only the specific categories show up in the particular .php file
and 2) posts relating to those categories only gets displayed through their respective php file.

Is this possible? how? I’m still on wp 1.22