The 7800GS does exist, but is very hard to find. Apparently, a few have managed to sneak out of the Nvidia labs. I know this sounds unbelievable, but here is a link to prove it:http://www.anandtech.com/video/showdoc.aspx?i=2616
This card performs exactly on the midpoint between the 6800GS and the 7800GT. Basically, it is a 6800 Ultra using the 7800 series core.