Reilable install - 2 HDD/CD/USB?
I have an old PC (Celeron; 512MB RAM; USB boot BIOS option present).
What is more reliable, to install pfSense on:
1. Two gmirror-ed HDDs. If one of them fails, we can boot from another.
If this old PC fails, we can move HDDs and network cards to another old one,
they are extremely cheap.
2. USB flash stick. Is possible also to use gmirror for USBs?
3. To run pfSense from CD and to store the configuration on Flash/FDD?
As you said, the hardware is old and could easily fail at any time. Likewise it could keep working for the next 5+ years. Also as you said, it's cheap, a similar system and you could easily transplant drives or whatnot into the replacement. Or use the replacement for spare parts and rebuild the original.
An add-in controller (any el-cheapo RAID card can do mirroring, just have to make sure it's FreeBSD supported) would possibly be a good investment. It would let you move the array to another machine even if it wasn't exactly the same (barring an architecture change such as x86 to x64).
As for booting from USB it should be doable. Name brand servers are coming with internal USB/SD/CF ports for embedded VM installs for things like ESX/ESXi. If a USB flash disk can run an enterprise grade server like that it should be reliable enough for a firewall. I'd get a micro sized one (usually sold for laptop owners that don't want a large piece sticking out), this way you don't have to worry about something hitting it and snapping it off.
Not sure about booting from the CD and utilizing USB/HDD for config files. You've still got the USB/HDD to worry about just as if the whole OS were on it. And it seems like a hassle to restore a config file each time you reboot the firewall.