my answer your questions are as follows:
i) when we talk about stored procedures which is on your mdbs (Management DataBase System software - is the database program like
mysql/mssql/oracle/sybase etc.. in you computer) server when you run
it or call it (execute) it will do the job. when you want to check first the data being entered before inserting the field value in your database i suggest 2 ways (especially in a web environment):
2) use php/asp/jsp/vbscript to validate on the server side.
using one of the two lessen your load to your database program.
ii) i don't how it will work for you, but trying will add to your job knowlege, and if you do back up first your current system in place and try restoring it so when ever your beta version of system
crashes you will know what to do.