Sharepoint is an amazing platform for business users. However There are some limits to how much you can stretch it. Following article sums up various numeric limits which a sharepoint based site can hit. Performance Guidelines for MOSS - Bobby Habib SharePoint & MOSS Blogging Space
Gist of the article (taken from article verbatim): Capture and review the requirements for the common cases.Feel comfortable that they are at least generally correct. Implement them. Get feedback from the users, after prototyping or implementing the solution. Analysis Paralysis and Agile Development | Tyner Blain