NAS = network attached storage. Network-attached storage - Wikipedia, the free encyclopedia
RAID is a way the drives can be set up. RAID0 is data striping, splitting up the data between drives to increase speed (reading from all drives at one time). RAID1 is mirroring, writing the same data to separate drives as backups.RAID5 is basically a mix of the two (much more involved, but that's it in a nutshell).
The first solution offers the most storage. If set up as a RAID5, it would equate to a TB, but be very well protected against drive failure.
The second is a flat TB. If it offers RAID, it would cut the storage by the type of RADI used (RAID0 = 500GB, RAID5 = 250GB).
The third is 640GB, but is a HP(?). You would have to look up the specs on it to see what it is for sure.
Without complete or mostly complete specs, that's all I can do... recap the options. From what I see, option 1 looks the best to me storage-wise.