After poking around a bit, I discovered how to determine what models a Dell Resource CD covers.
Insert the disk, and say NO to opening Add/Remove programs. Right click and select explore. Open the RESOURCES directory. Open the RCD.mdb database (you will need Microsoft Access); say OK to the Read-Only warning. Open the PlatformIDs table. Models covered by the disk will be listed in the table along with their Machine IDs.
There is NO correspondence between the S/N or any other number on the external features of the disks. I have found two identical resource disks, right down to the revision number, that contain drivers for different Optiplex models.
Go figure. Anyway, I figure this is good info for a lot of you out there. BTW, Access 97 works just fine for this, so don't spend a fortune getting the latest.