The following will force it to see the new hardware.

1) Shut down the computer.
2) power up in open firmware. (hold "cmd-alt-O-F") You'll see a blank screen with blinking cursor.
3) type "set-defaults" and hit return.
4) type "mac-boot"
and hit return.

Your cpu should now reboot and it should list what it found in its pci slots.

If it does not work, make sure that your SCSI card is properly seated in the PCI slot, verify your cabling and termination as well as SCSI ID's of the connected devices.