Depending on the P4 chip, you may be able to run the amd64 version of FreeBSD. You'd have to look at it under FreeBSD, though, to see if it supports
LM (note the AMD features below, taken from my FreeBSD 8.2 box):
Code:
CPU: Intel(R) Pentium(R) 4 CPU 3.00GHz (2999.92-MHz 686-class CPU)
Origin = "GenuineIntel" Id = 0xf65 Family = f Model = 6 Stepping = 5
Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
Features2=0xe59d<SSE3,DTES64,MON,DS_CPL,EST,TM2,CNXT-ID,CX16,xTPR,PDCM>
AMD Features=0x20100000<NX,LM>
AMD Features2=0x1<LAHF>
TSC: P-state invariant
While you shouldn't need to tune anything, you can guarantee better performance if you do some manually ZFS tunings. Depending on what else you will be running on the box, you will want to limit how much RAM is used by the ZFS ARC. For example, my home "server" runs Samba, NFS, DLNA services, torrent downloading, etc, so I limit the ARC to only 768 MB (out of 2 GB) to leave room for the other services.
You will also want to make sure you disabled prefetching (it should be disabled automatically, but it's good to manually disable it as well).