Go ahead and use the lowest powered processor for your firewall. PFSense typically doesn't use much CPU time. Not sure how this changes with a squid proxy, but with a healthy rule table and 20-30mbit of consistent web traffic (40 webservers + 5-8mbit consistent email traffic), we run about 25% CPU usage on a P3 850 w/ 512MB of RAM.
We've considered going to higher performance machines, but the old dell's that we have don't have a power switch that can get accidentally turned off, and they just chug along with not a care in the world. We typically go over a year or more between reboots, and the last two reboots have been because our upstream provider was having issues. We power cycled the system just to make sure that it wasn't on our end.