Multiple checks on submit

Hi,

Could somebody please help me ?

I,ve created a form which contains ...
- a list of radio buttons
- a couple of textfields
- a create user template

I would like to the delete the "create user" button and add a new button
which...
- have the same functionality as the "create user" button which checks the
username etc...
- stores the other fields in an other database
- does some small tasks...(some calculations etc)

Does anyone have some great advice how to approach this ?

Thanx

John