The quick and easy answer is to look at the Database Accelerator Suite of Webservices. http://www.qdabra.com/proddetail.asp?prod=QDBXL1
With DBXL you can convert your existing forms and have them submiting to SQL in under an hour. This also preserves the original document for storage, retreival, searching, and controlled access. It also comes with webparts for SharePoint so that you can still use SharePoint to list them.
Otherwise you have to create your own webservice or you would need to connect directly to the database. If you are just going to submit the data to SQL, without a webservice, you pretty much are stuck recreating your form. You have to create the form with a database connection initially and then you may run into some limitations.
You can email me directly if you have more questions.