The advice I got a couple of years ago was to go for 7200RPM SATA3 disks, make sure they are branded as enterprise quality (worth paying for) and buy them from different manufacturers.
Then mirror the disks and hopefully one disk will die before the other and
thus you get some long term reliabality.
I bought two 500Gb disks:
WDC WD5002ABYS-01B1B0
SAMSUNG HE502IJ
which have been solid and fast (so far).
-Steve
