best way to design site...

Hi everyone on the list, this is my first post here... Quick background to question: I am writing a BBS system for the company I just joined with PHP4 and MySQL. This will be running on a Linux box. Quick question: What is the best way to approach the issue of designing the system. Would it be best to have a few fairly large php files that do all the work, or would it be better to have many smaller scripts each with there own job. For example, the system will offer the choice to switch from nested view to flat/ chronological view. This could be done by having a php script for each view, or just one bigger script, and just pass a variable (view=flat/tree). I would like to make the script as efficient as possible. So which method would be better ? I am wondering if there are issues regarding opening and closing files, or would one file sit in the cache better ? This site will be hosting over 3000 BBS so I would also like to ask is it better to have one database, with 3000 tables, or split the BBS into perhaps a dozen tables, but with many more rows, and an extra field identifying the BBS. Will