Neoseeker has published a review of BFG's 7800GS OC AGP card.

We can see that when going from the 7800 GT PCI-Express to a reference 7800 GS AGP, NVIDIA has dropped the core clock speed by 25 MHz, bumped up the memory clock speed by 50 MHz (100 MHz effective), reduced the number of vertex and pixel shader pipes, and very surprisingly, cut the amount of ROP pipes to half. Doing some rough calculations, we can expect a reference GeForce 7800 GS to take about a 15-20% performance hit when compared to the 7800 GT. Please note that the sample sent to us by BFG Technologies belongs to their "OC" line-up, which enjoys slight bumps in core and memory clock speeds, as well as snazzier blue PCB and cooling systems -- those are the numbers you see in brackets.

BFG 7800GS OC AGP Review