But don't these Raid drives need exact clone hardrive specs to work together? If so, it's not going to be such an easy task finding a spare drive after a year or so when specs have all moved on etc.
You get best results if they are either
1) identical
2) failing that, a drive of equal or faster speed, with identical drive geometry. (e.g. a single 80GB platter instead of 2 40GB platters. If the physical layout of the drive is too different, there may be too much of a difference in the time it takes the heads to travel from sector -x- to sector -y- between the drives, which means the controller has to wait for all of them to get to the proper location.
3) If you can't get either, get the fastest drive you can find, to limit the chance of delays.