Having sectors go bad is normal for modern hard drives, both because of
their high density and their large number of sectors. If sectors go bad at
a low rate, it is simply a waste to replace the drive with one that will
hopefully perform the same way and not just fail early.
That's my way of thinking, too. They have some in-built level of error
handling, for that reason. So long as the faults are insignificant,
you're okay. I'm certainly not keen on coughing up $150 for a new
drive, *just* because two sectors stopped working, for instance.
Modern drives have spare sectors that are mapped into use internally if
bad ones are detected. However this is normally transparent. If SMART
says you've had a few errors that were recovered I wouldn't worry about
it. On the other hand if you are seeing errors at the OS level it
probably means you are out of usable spares and the drive is about to
go. Most manufacturers have an on-line warranty check - if it is still
covered, get a replacement. If it isn't, you can probably get twice the
space for the same price now.