I would back-up critical data on the existing hard disk first.
To reduce the possibility of an error, I would not install my old drive on the Dell until after the drive that came with the Dell is fdisk’d and formatted.
When using ATA/33 hard disk drives and a 40-conductor flat cable, it makes no difference what connector on the cable is connected to what. For example, one can plug the connector at the very end of the cable, where the two drive connectors are closer together, into the motherboard, and plug the middle connector into the Master drive—if it will reach that way. It does make a difference what connector is plugged into what with an 80-conductor ATA/66/100 cable and drives.
One can change which partition is active after the fact with fdisk.
You may have to go into the CMOS Setup and set the drive parameters. If it has an Auto setting for drive settings, try using it.
If your existing drive has a disk overlay installed such as Disk Manager of Easy Drive it probably should be removed before installation on the Dell.