Firewall-Hardware or Software?

We are a small company and use 2 NT Servers, one as Mail Server & the other as a File & Print Server. We also use a broadband connection and a roaming facility from two separate service providers to access our mails securely while on travel or from home. We don’t use any firewall at the moment and are keen to know whether do we need one. If yes, which would be the ideal one – Software or Hardware or a combination of both. Please reply with some technical inputs as well, Thanks.