The NVIDIA GeForce 7800 GS really exists as AnandTech was able to get their hands on one of these cards. They have published some more information about thus videocard.
The card is a 16 pipe replacement for the GeForce 6800 GT and 6800 Ultra and based on a couple of benchmarks it looks like this card performs right between the GeForce 6800 GS and the GeForce 7800 GT.
The card we benchmarked came with a 375MHz core clock and a 1000MHz memory clock. Even with an entire quad disabled, our internal testing proved that the card was ample competition for ATI's X1800XL. Currently, we are working on enabling the disabled quad and/or overclocking the 7800GS up to 7800GT and GTX speeds. With a good price point, the GeForce 7800GS could easily become 2006's rendition of the Radeon X800GTO^2. A 16 pipe version of 7800GT would easily best a GeForce 6800GT or 6800 Ultra (at least on paper), but with current drivers we've already witnessed higher performance than that.