Been running m0n0wall for about 5 years on an Alix board and for many years before that on a P3 500. Never had to reboot it once. When my WAP54g started to flake out I replaced it with an Ubiquiti UniFi UAP which blows the old Linksys WAP54g away. The old 54g could barely push a signal 2 floors down to the kitchen, living room, and the basement was a dead zone. Now I have a strong signal throughout the house. It has a boat load of features and is 70 bucks on newegg.
I did not know about the APU.1C, thanks for posting that. The only thing is m0n0wall is not SMP aware, though it will run on an SMP system like the APU.1C. You are better off running pfSense which has SMP and 64 bit support. That board should have no issues pushing over 100Mbps while running multiple services and VPN encryption. My only concern is the use of Realtek gigabit chips. I wish there were Intel nics on there.
Soekris Engineering makes some pretty solid hardware but it comes at a price. The net 6501 has up to 2GB ram, single core Atom 1.6Ghz, 4 intel Gigabit ports, SATA, mSATA/USB, mPCIe and up to 2 PCIe x1 slots for actual PCIe cards. A top of the line 6501-70 and case will run you $470 without any accessories (storage, power supply, etc.). Before the 6501 came out I was going to buy a net 5501 but I couldn't justify the price so I went with the Alix.