Try common of the shelf PC hardware.
We have been running OpenBSD on old AMD dual core MBs for quite some time now. The machines are fitted with an intel quad port GB adapter. but otherwise there completely standard PC's. We have a bunch of these MB's and every component is easily replaceble. We have two identical machines running side by side, so when its time to upgrade, we yank the cables from one box to the other. We have been contemplating to use CARP for failover, but i'm a firm beliver in simple things (the importance of KISS can't be overstated).
Throughput and stabilty is great. We de a lot of webhosting and have a lot of S2S IPSEC tunnels.
Furthermore the OpenBSD boxes can do some tricks that the trained monkeys, with their Checkpoint, cisco, juniper and so on at our customers sites , typically have never heard of (like port based ipsec routing for example).
If you have the knowhow, an "enterprise" firewall with all the service agreements, licensing costs and other thievery is just money out the window.