Hi JeepFan,
I did some digging and that box appears to be a special "custom" model that Dell made for a specific customer...the type of thing where someone like Facebook or something needed 20,000 nodes and Dell was happy to oblige. That means that model doesn't even exist of Dell's support page so there's no BIOS updates or anything like documentation out for it.
That doesn't mean it wouldn't be ok to use for a FreeNAS box, but it's certainly something to take into consideration. From the posts I found the other major caveat is that the thing is vacuum-cleaner loud, so keep that in mind as well.
Honestly, the box looks to be complete overkill if all you want is something that would use 4 drives (the internal limit)....if you are looking for something to hook up to an external JBOD box via a connection provided by external ports on a SAS controller it should work, assuming the BIOS hasn't been hobbled is some way to limit your expansion card choices.
If you are looking for something to use at home and think this would be a good cheap server with ECC support you might want to reconsider.
-Will