the dell one isn't a NAS in the normal sense, it is a proper full file server computer running windows server 2003, to configure it you would probably need to attach a monitor keyboard and mouse to it.
the readynas is a simple NAS storage unit, probably running Linux of some sort and will be configured via a straight forward web page from your normal computer.
If all you want is network storage i would go for the readynas, if you want a full blown server then get the dell.
p.s. probably about �400 - �500 of the cost of the dell one is the license for windows!