I am using the Western Digital WD20EADS 2TB drives in my DNS-343. I have four (4) drives setup as RAID 5 EXT3 using 1.03b70 firmware. So far, it is working perfectly. The formatted volume reports 6 TB, which is correct for a RAID 5 solution.
Now if only I can get bonjour to work without having to funplug:
http://forums.dlink.com/index.php?topic=5659.0