if you are so keen on thunderbold and had bought all your devices already with thunderbold , then sure you got money leftover for a new iMac with thunderbold , so sell your ancient Mac before its to late , as many will sell now
Since Thunderbolt uses PCI express, I wonder if the Airport card could be removed and a Thunderbolt card could be installed. It would require some modification to make an extra external port, but it sounds like the only option for older iMacs.