FreeNAS cannot "manage" vSphere - you can use FreeNAS for storage though.
Is this for a home lab setup?
As far as FreeNAS goes - your hardware setup will depend on what you can afford, how you want to set things up, and what kind of performance you require.
I have found this discussion useful:http://forums.freenas.org/showthread.php?9977-Poor-ISCSI-performance-on-FreeNas-8-3
I assume that you will want to use ZFS over UFS? Also assume that you will present whole disks as iscsi targets on FreeNAS rather than partitions? Will you mirror drives or create a zpool? How many TB of storage do you want for vSphere? Assuming gigabit network st least - and maybe jumbo frames? http://doc.freenas.org/index.php/Hardware_Recommendations
The phrasing of your question is confusing, and with so little detail it is hard to provide specific advise - hope that this helps.