Ok, you will want to do a Green+Red setup. Make sure on your router to disable DHCP, and set it within the same iprange as the smoothwall, but outside of the DHCP range [192.168.0.x] (not required, but less of an annoyance).
In the setup, just make sure to set your Red interface as DHCP (im assuming that your modem connects automatically, and you are assigned a dynamic IP from your ISP. If not, then set it accordingly, being Static, or PPPoE.)
Once you go through the setup, when your smoothwall is started again and shows the login prompt, you can go to your normal desktop computer and just type in
http://smoothwall:81 into your browser. The username will be admin, and whatever admin password you specified during the setup. Everything else should be pretty self explainatory.
Feel free to ask any questions if you need more help. I recently setup one here at my house using PPPoE on the red interface, and one at work using DHCP on the red interface, so im proficient in the setup process.
Also, as to your questions about the advantages, I can give you an example of why I did it at my house.
My previous setup was like such:
DSL Modem----->D-Link DI-624 Wireless Router-------~>Computers via Wireless.
Every few hours, I would have to manually unplug and replug in the power to the router, because it would drop the internet. Still show as being connected, but nobody could do anything on the internet. However, the network itself worked fine. After getting tired of this, I setup a smoothwall using an old P3 system I had under my bed. Now, with my setup being:
DSL Modem----->Smoothwall----->D-Link DI-624 Wireless Router-------~>Computers via Wireless
I have gotten a result as such, with my internet:
Connected (8d 5h 51m 32s)
With my work scenario, it was on a basis of such:
Cable modem--->Linksys Wireless/Wired router--->8 port switch---->Computers
The issue here was that at random times, the router would stop allowing the computers to access the webserver that our website is hosted on. It makes no sense at all, because the webserver is hosted remotely. However, we could access everywhere else on the internet, except our website (and its ftp services). In order to gain access again, we would have to unplug the router power, and plug it back in. Finally getting tired of this, my friend and I setup a smoothwall, to where the setup was like such:
Cable modem--->Smoothwall---->Linksys Wireless/Wired router--->8 port switch---->Computers
Since that time, we have not had a single issue with not being able to access our webserver, or the features it provides.
So, I
HIGHLY recommend smoothwall to anybody.