The hardware firewall will protect your networked PCs from inbound attacks. What it will not do is prevent a program (say mail prog or other "phone home stuff") from sending to the outside world. If you get infected by something that sends info, it will just be sent.
So, you only need a software firewall to prevent outbound traffic you don't want to be sent. A software firewall such as Zone Alarm could be set to monitor outbound only, but if you are confident your own security procedures are stringent enough to prevent infection in the first place, then you could do away with software firewalls - I have.