When setting up new disks, follow the OpenBSD FAQ for setting up a mirror. Steps are roughly: after bioctl step to get a new softraid partition, then you do: disklabel, newfs, and then edit /etc/fstab to determine mountpoints. (I think)
A Few of My Favorite Things About The OpenBSD Packet Filter Tools
OpenBSD Router Guide
PF Setup
pfctl -s info will give you stats on how much data is flowing through PF, and uptime.