Where should I write business rules and data validation if I want to apply some client-server and n-tier concepts?

Should I use stored procedures?, Table´s field and record validation rules?

I don´t need to build a robust 3 tier app, but I want to begin separating data management and UI, so I would like to know good practices wich allow future scalability.

Let´s say: if the user must provide at least two fields (code + name) to be allowed to save a record, where do I put this validation?

Users have been using programs where they have buttons to navigate through the records (go next, prior, first, last), and perform incremental search, and even determine the sort order. How can I provide this environment using CS and n-tier techniques?

TIA.