Well, I do have to say, if it ain't broken, why fix it? But if you really want to switch away from OpenBSD, I agree with staying away from Raptor. I personally would stay away from Checkpoint as well. I haven't seen many problems with the Cisco Pix systems, and a few of our firewalls at work are in fact Pix.
If you really want to check out Linux firewalling, see here:
http://www.linuxsecurity.com/feature...netfilter.html
It gives some good information on iptables (the newest and greatest from the 2.4.* kernel). Iptables give you many many new abilities over previous incarnations in Linux firewalling.
You can spoof your true operating system and version, a move in the direction of stateful packet filtering, and more! If you decide to go the way of Linux, I think you'll do fine, provided you study up and do some testing before placing it in production.