A forum search for "fitpc" turned up this note: http://forum.pfsense.org/index.php/topic,15132.0.html suggesting at least one user has successfully installed pfSense on a fit PC. The same search turned up another user who apparently gave up trying to install pfSense on a Fit PC.
I have no experience with Fit PCs. However I have been running pfSense 24x7 on a mini-ITX system with a 1GB solid state disk module for almost a year. The module is a Transcend TS1GDOM40V-S. It plugs directly into the IDE connector on the motherboard. There are 44 pin variants to replace notebook drives. I bought it from http://www.memoryc.com, current cost 14 euros and they ship internationally. This won't be suitable if your pfSense needs lots of disk capacity (e.g acts as web cache) or is a heavy disk writer. Compact Flash cards are another low power alternative and from reading the forums it appears there are a number of pfsense users who have chosen that option.