If you're going to change the form action based upon values set in the form itself, it would be easy to do with JavaScript. You can change the action property of the form dynamically.

From www.devguru.com:

action Property
This property specifies the URL address to which the data
gathered by the form will be submitted. An email address
can also be specified using the 'mailto:anybody@anywhere.com' syntax.

Syntax: object.action = URL