というわけで、実験してみました！
デフォルトでインストールしただけの状態ですが、128MBでの起動できました。
$ cat /boot/loader.conf
kern.cam.ctl.disable=1
-------
Copyright (c) 1992-2012 The FreeBSD Project.
Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
The Regents of the University of California. All rights reserved.
FreeBSD is a registered trademark of The FreeBSD Foundation.
FreeBSD 9.1-RELEASE-p4 #0: Mon Jun 17 11:42:37 UTC 2013
root@amd64-builder.daemonology.net:/usr/obj/usr/src/sys/GENERIC amd64
CPU: AMD A4-3300 APU with Radeon(tm) HD Graphics (3000.11-MHz K8-class CPU)
Origin = "AuthenticAMD" Id = 0x300f10 Family = 12 Model = 1 Stepping = 0
Features=0x783fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,MMX,FXSR,SSE,SSE2>
Features2=0x80802001<SSE3,CX16,POPCNT,HV>
AMD Features=0xea500800<SYSCALL,NX,MMX+,FFXSR,RDTSCP,LM,3DNow!+,3DNow!>
AMD Features2=0x3e9<LAHF,ExtAPIC,ABM,SSE4A,MAS,Prefetch,OSVW>
TSC: P-state invariant
real memory = 134217728 (128 MB)
avail memory = 104673280 (99 MB)