I have some experience with that hardware and concur with Eric; Uses too much power. and, contrary to the main description on the add, I've never seen a 2950 with 2.5" disks. Somewhere else in the add it says 6x3.5" which is true and, for a FreeNAS system, a bit sad. Actually, for any storage, as a matter of fact.
8 GB is also the bare minimum in terms of RAM, so... Just think again.
As paradoxal as it might sound, a PE2900 will accept more ram, have at least 8 disk bays (up to 10 with with the flexbay thing), but uses pretty much the same power. It is about 400W, continuous.
If you are REALLY going for those, make sure they are Generation 3 (Gen 3, or Gen III). You chance of getting a 2 x quad core processor is better, and this is especially good if you will play with compression, which I recommend.
Now, if you have the choice, go T610, rack mount. 8 disk bays as well, but HALF of the power consumption.
Unlike what Eric said, it is not that the SAS controller is useless. You need the SAS controller, and it will even control SAS and SATA disks (don't mix them, though). The RAID controller is not recommended. (I am pretty much this is what he meant) . Indeed, in order to play right with ZFS, and use the most from its features, do NOT use the RAID controller in RAID mode. The lazy man option (mine) is creating 8 volumes, one for each disk, and let FreeNAS/ZFS handle them in a RAIDZ2 set. There are pros and cons to that. I'll let you browse the forums about this topic.
When it comes to Dell, you usually do not have the option of using the PERC in JBOD mode. Older Dell-branded controllers normally won't have it. PERC 5 and 6 are two of those. Newer controllers may include JBOD. Do you homework before buying, but, for a learning system, it may not make too much of a difference.
Remember: RAID5 is dead. If you care about your data, go RAID6 (RAIDZ2) or better. Now, the good thing is, those servers usually come equipped with ECC RAM. At least you won't have to worry about that.
Just to give you some background; I am not "just saying" these things. I am actually decommissioning 7 PE2950 with that very same config, because they cannot handle enough memory or disks. Not to mention that I can power 2 R710s with the power used by one PE2950.
Again, unless this is a box for learning the stuff, and ultimately you are not going to pay the utility bill, I'd go fo something else.
