Network and Internet security are getting more popular these days.
While security is crucial, but you will still have to weigh how much you are going to invest on it.
Do I need a firewall ?
In my personal opinion, a NAT box is sufficient for protecting your home computers/network, but not for a office/company.
NAT is a feature that are found in almost all routers.
And if you feel you are in doubt, ask yourself when you are getting a firewall ...
"Where and what should I protect from the internet ?"
What is Router and NAT?
A router does routing for network traffic, while routing is the act of switching packets between networks
NAT - (Network Address Translation) - is primarily a security measure that translates a public IP
address into private IP addresses that are used within an organization's network. Using a NAT
device on the edge of a network protects the individual workstations and servers from malicious hacking. ... extracted from wikipedia
What is Firewall
Firewalling is the act of providing access control between two endpoints,
whether those endpoints are entire networks or single devices.
Conclusion
In conclusion, as long as you have a way of controlling network access, I would consider that you are FIREWALLED. :-) ...
And with the advance of technology, manufacturers are now squeezing more technology within a box, and you might already have
the firewall features within your router.