I believe the raid on this is also software based not hardware, but some people have then purchased a RAID card to go in one of the PCI-X slots and ignored the onboard raid. However a friend of mine is looking to raid 5 his, I'll have a word with him to see how he is going to do it?
That'll be me then! I used MS Server 2008 R2 and software RAID 5 to make 4 x 2TB Samsungs into 6TB. Seems solid as a rock, with shares working well across XP / Win 7 and OSX platforms. Decent read / write speeds, that seem to keep up with the requirements of multiple machines connected to it, including a HTPC running XBMC.