If creating an FreeNAS VM, choose Windows Vista (32bit) as VM Machine, as it will use the Intel E1000 NIC which does give you about double network performance.
DON'T FORGET TO CHANGE BACK VM TYPE TO LINUX (32bit) BEFORE POWERING ON!!!!
Taken from: http://www.network0.org/2008/11/18/on-freenas-and-esx-performance/