7900gs = overclock 7800gt.
Slightly wrong there. It is produced on a smaller process and has 1 extra vertex shader i think.
Nope, 7800gt has 20 pipes and 7 shaders, same as the 7900gs, but yes, it does use a 90nm core instead of the 110nm 7800gt
Yeah i wasn't too sure about the vertex shader thing, thanks for correcting me!