Thanks to everyone who has given me an answer. I feel much more on top of things now. However, the instructions to move the present drive to the lower bay before installing the new one in the upper still present a puzzle. The drive I have clearly seems to be SATA, so it can’t be because of the IDE issues that Jake Jones describes. As everyone seems to agree that moving the first SATA drive is unnecessary, I wonder if, perhaps, in creating the manual for this model the Dell editors could have copied some of the material from an older manual for a model with IDE drives. Although the pages preceding it do mention SATA, the one where the instructions about moving the old drive to the lower bay do not say anything that directly specifies the interface type.
Anyway, after a bit more googling around, I have discovered one more issue. According to this page, my motherboard (on the Dell Dimension 8400) only supports 150 mb per second SATA (that’s the same as 1.5Gb/s, right?), and will not work with drives with a higher transfer rate. The vast majority of the drives available at Newegg seem to be 3.0 Gb/s or higher. This sitetalks about resetting jumpers on a WD 3.0 Gb/s to make it run at 1.5Gb/s. Should that be possible (and reasonably easy) on any 3.0 Gb/s drive, or would I be best advised to search out one that only runs at 1.5Gb/s (the few that do exist do seem to be a bit cheaper)?
Also, if, one day, I want to remove the new drive and put in an enclosure, most likely for use with the new laptop I hope to get if I ever have the money, is it going to be a disadvantage if the drive I have can only transfer at 1.5Gb/s. I imagine that if the enclosure is going to connect through USB or IEEE 1394, then it won’t make any difference, but with eSATA it probably would - but that is just guessing.