There are several online lead management systems, such as Salesforce and Leads360. Salesforce is the industry-leading customer relationship managment system (CRM), and we recommend it. When a lead completes your online form, their data is stored securely in the Salesforce database where the lead is then distributed to one of your sales persons.
Salesforce can be configured to integrate with most back-end systems so it should be able to pass data to your back-end systems when a lead decides to close. You purchase Salesforce on a per-user bases, and it ranges from $10 to $125 a month per license.