Samdaniels:
Casey's FireStudio Lightpipe suggestion is just about the perfect solution to do what you need to do with your iMac/D8B combination. Unfortunately, it has been discontinued. You will have to turn to eBay or something and look for a used unit. While you're looking for that, also look at the M-Audio Profile Lightbridge. Also discontinued, but basically the same featureset.
And before buying one of these blind...please investigate the various forum boards (Presonus, M-Audio, Gearslutz...etc.) on these units to see what problems other users may or may not have had with them....particularly driver related issues.
The MOTU 324PCI card won't do you a bit of good with your iMac.