For most three-dimensional applications, like games, the 8800 GT renders roughly 10-20% more frames over a given duration than the 8800 GS (see this analysis for more information).
However, for Pro Apps the GS is surprisingly superior (according to this analysis). I guess Apple/Nvidia must've optimized it for Core Image operations a little more than the GT.
As for how they technically compare, here's what the 8800 GT has:
112 stream processors
600MHz core clock speed
900MHz memory speed
256-bit memory interface
up to 512MB GDDR3 video memory
And the iMac's 8800 GS is technically a rebranded 8800M GTS (according to at least one source; iMacs use notebook components, after all). The specs for this card are: