It can be run off a CD and a floppy to save settings to, however, I recommend a writable medium so you can configure it easily and not have to worry about losing settings. You can get a CF card for cheep, currently the image is 5.32 MB so anything larger then 8 MB will do, and an IDE to CF converter for about $10.
Mine system never tops 10% CPU usage; you can run this on a Pentium I.
you can add a wireless card to it, but the list of supported cards is slim, however, you can disable DHCP on the BEFW11S4 and use the LAN side and the AP, skipping the WAN and it will work great, that’s is what I do. The m0n0wall handles the routing, while the BEFW11S4 acts as an AP
M0n0wall has a large following and great guides to get you going.
http://m0n0.ch/wall/quickstart.php
Once the basic setup is done with a keyboard, you can use the web interface, which makes everything nice and easy.
My setup:
the modem connects to the WAN port on my m0n0wall box, then the LAN port plugs into my gigabit switch, which directly connects to my system, my file server, and to an 8 port 10/100 switch, this switch leads to all other systems in my house and to my wireless router, which has DHCP disabled, the wireless enabled, with WPA2.