Im not advocating less spindles, just more space. Remember that disks get slower as they get fuller and that you'll probably want to back up to disk. Cache does offset spindle count to some degree, but I advocate being conservative.
We've lost one disk since our SAN has been up...a year? Hot spare handled it fine. We're using the 15k rpm drives which run hotter, but still you can expect a failure. Main thing is to make sure you know when the first one fails.
Andy