http://www.freebsd.org/doc/en_US.ISO...-hardware.html
Both FreeBSD/i386 and FreeBSD/pc98 require a 486 or better processor and at least 24 MB of RAM. You will need at least 150 MB of free hard drive space for the most minimal installation.
Note: In case of old configurations, most of time, getting more RAM and more hard drive space is more important than getting a faster processor.