The NAS box presents its storage to a networked PC as a logical volume, irrespective of how the disks are configured in the NAS box. The networked PCs have no visibility of how the disk(s) in the volume are arranged.
A bit of a sweeping statement Saab, most NAS' can be partitioned into logical volumes and some even support iSCSI for mounting of remote disks locally (ideal if your using something like SQL Server which typically will not allow installation of database devices on remote disks) - these are stored as preconfigured files on the NAS.
As for disks, The Seagate barracuda is a good drive as is the Samsung Spinpoint F1 (which is benchmarked slightly quicker but your constraint will be the network not the drive). Either will do and the price is virtually the same (a quick google threw up the F1 £1 cheaper at £56) so not much in it.
Personally for speed I would install two drives mirrored in my local machine and then get an eSATA external drive for backing up which can then be removed off site (eSATA giving you similar speeds externally than externally but cannot be used over great distance).