Astaro would be my first choice (currently running it). It's a very solid firewall appliance like distro, though "limited" in the sense that you cannot really add-on other software. Even though you cannot add-on anything to it (unless you really, really, really know the ins and outs of Linux), it has plenty of options to allow you to customize its function for the network your protecting, and imo, is the best at it.
pfSense would be my second choice. A stable firewall with lots of built in options, as well as the ability to add-on more.
Can't say much about IPCop, but it seems to be another good distro.
Dunno if its improved any since I last used it, but last I read about it the consensus still was to avoid Smoothwall. It has some quirks and bugs that really brings its function down, and it hasn't been able to catch up or keep up with the other 2 distros above (IPCop, pfSense).
[Edit] Just saw you mention the USB nic. You'd have to check if the distro supports it, as many do not support USB nics fully, if at all. You're better off with a PCI/PCIe NIC. [/edit]