My smoothwall setup is pretty basic. I have an atom ITX board with two gigabit NICs, 2gb of RAM and an 8GB SSD. I like this setup because it doesn't use very much power and there is only one small fan.
However previously for about 4.5 years it was running on a Pentium II 333MHz with 384MB of RAM and a 8GB spinning disk HD. It worked fine on that hardware, the only reason I upgrade is that the CPU fan and the power supply fan were starting to give me trouble. (And I wanted to build and play around with those ITX atom boards.)
The software is pretty easy to use, as long as the NICs are supported by Linux you shouldn't run into any real difficulties. It supports up to 3 different network zones. I'm only using one zone. The other two are for a DMZ and wireless. The idea being you can keep those isolated from your primary network for better security.
If you want to play around with it, you can run it in a virtual machine. And they even have premade VM images too: http://www.smoothwall.org/download/g...al-smoothwall/
There are also install guides and such: http://www.smoothwall.org/download/
but at least to me it is pretty straight forward. Once you have it up and running there is a web interface that can control everything. The only time you ever need to SSH into it is if you want to install a mod. There are a lot of mods: http://community.smoothwall.org/foru...p?f=26&t=23845