Honestly, I'd look at the 4-drive NAS boxes. It's the 'optimal' for space vs. data security. With a 2-drive, you've got either RAID1 or RAID0... the space of one HDD, or total data loss in the event of drive death...
I've had a Synology DS411j for some time now (about 18 months, I think; the last year has been a bit hectic) and it's doing grand. I am, in fact, debating getting a DS413j to act as a pure media server for family. Running Synology's take on RAID5, transfer speeds are pretty good (in both directions) although Window's stubborn insistence in forgetting the NAS password every is annoying. Linux remembers it, though, which means XBMC is OK.
I like the idea of the upgradability of a custom built NAS running FreeNAS or similar, but using 'old' kit is going to mean it draws tons of electricity, and buying new low-power kit means it'll cost a fortune.
I've had good experiences with Synology, though.