In the Dell Optiplex (and a number of other systems), there is no means in the BIOS of assigning an IRQ to a PCI slot. Each available PCI slot appears to be "hard coded" to share an IRQ with a built in device, such that when the IRQ is changed for a PCI slot while attempting to resolve an IRQ conflict, both internal device IRQ and PCI slot IRQ change at the same time. The result is that the IRQ conflict continues to be an issue.
The recommended method of resolving an IRQ conflict in this case is to power down the computer and move the PCI adapter to another slot. Though the IRQ may still be shared, this time it may be shared with an internal device that does a better job of it.
Related article: Resolving Hardware Contention, IRQ or Memory Conflict with PCI Adapters