I have a very similar setup with two pfSense 1.2rc3's setup in a failover state. They are running on Dell 1U servers with 3 NIC's and have operated quite well for the last 3 months with about twice the number you have stated there. One problem I did run into was during the migration to pfSense we also migrated ISP'; during that process we use proxy ARP to use the IP addresses from the other ISP to be used on pfSense. Let's just say it didn't go smoothly, but everything is operational and we are happy with the cost savings.
Curtis
