I've done this in the past with a dual P3 box (only one was used by pfSense at the time) and it would route 10 Mbit with low CPU utilization (5% from memory. )
This time around I went with a netgate appliance. This one, actually: http://store.netgate.com/Netgate-Hamakua-1U-P229C84.aspx
Solid state, rack mount, the Celeron processor has enough oomph to do everything I need to do, and 4 gigabit ports plus the 100 Mbit WAN port give enough flexibility to separate out all my networks without resorting to VLANs. All I can say so far is that it doesn't appear to get hot – I'm still in the process of getting my new colo space up and running and can't give more thoughts than that.
I have no idea how the price compares to building your own, though; with a failover pair the increased reliability one would expect from solid state is less of a benefit as well.