Well, from personal experience I can recommend Vyatta, which is a Linux based Router, Firewall, VPN and can do much more. Vyatta also sells the hardware and software.
But my personal favorite is pfSense. pfSense is a free, open source customized distribution of FreeBSD tailored for use as a firewall, VPN appliance and router<taken from the pfSense site>.
You did not say anything regarding the expected load on the system, will the system be for a large company, small business, or personal. You could buy a server from a vendor with no OS, or use a desktop. If this is for a small business I had good luck with the ALIX system boards.