Supermicro X8SIL motherboard
Works wonderfully with a Xeon L3426 CPU (HT turned off in BIOS). 4GB ECC RAM, 4 port Intel gig-e card attached for 6 ports total, 40GB SSD drive for reliability. Complete box is under 80W for power consumption and under $1000 to build.
So far with NTOP, BandwidthD, Squid, 100 OpenVPN mobile users, 12 ipsec site-site, 4 IF running, 29 VLAN's and some fairly complex rules we've not hit 15% CPU usage.
Easily replaced a $35,000 ASA 5580 while adding a lot more functionality.
Very happy with pfSense - wonderful project! Just one little routing issue I am facing (in routing thread) but I am sure that's me trying to configure the thing like a Cisco ASA.
Out of curiosity, why did you disable hyperthreading?
It was recommended to do this under FreeBSD 7 as it doesn't differentiate between HT and real cores. Supposedly 2.0 on FreeBSD 8 will correctly do this as it was added to FreeBSD 8.