I've got an older 20" G4 iMac, so the "positioning" of the AE card is different. BUT, I still don't get the best reception with it. I'm not sure how the new iMac's work, but in the case of your laptop, there are antennas that run through the plastic shell on either side of the display (hence the better reception).
The only thing I could suggest (and what I did), was to re-position your router!