The processor difference may have been a factor, as well as the resolution. You might want to take that into account.
Okay, comparing the iMac to the 17" uMBP.
iMac: 2.4GHz Core 2 Duo, 4GB Ram (Back Then), 256MB ATi X2600, 1920x1200
MBP: 2.93GHz Core 2 Duo, 8GB Ram, 512MB nVidia 9600M GT, 1920x1200
Back then, the most intensive game I played was CoD: WaW. I was able to run the game at 1920x1200 @ all low and get over 30-45+ fps stable on the iMac. I got ~15-30 fps on the MBP with a few frame drops around.
So maybe it wasn't a entirely fair comparison here. So lets go with the MBA Rev C, iMac and Core Duo MBP. Specs as followed:
MBA: 2.13GHz Core 2 Duo, 2GB Ram, 256MB nVidia 9400M
iMac: 2.4GHz Core 2 Duo, 6GB Ram, 256MB ATi X2600
MBP: 2.16GHz Core Duo, 2GB Ram, 256MB ATi X1600
The game I played mostly recently on these systems (before I sold the air) is Star Trek Online.
MBA, 1280x800 w/ all low and ~20 fps.
iMac, 1920x1200 w/ all low got ~30 fps
MBP, 1680x1050 w/ all low got ~35 fps
From my experiences, the mid range cards, the ATi models seems to perform slightly better then the nVidia counterparts.
Note: This is from my personal experiences.