|"I want to add a 320 G Western Digital hard drive to a Dell Dimension 2400. I have added a PCI Connector Card"|
You can have 4 IDE drives on the existing 2 IDE channels already.
If you insist, you can install the PCI card and follow the (badly or poorly written) instructions.
It may or may not be supported on reboot. Might have to install drivers depending on card.
You should not have to fool with bios. The system IDE is most likely the first active bootable partition.
"Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10