So what's under the hood? It'll no doubt surprise you to know that it's Nvidia's brand new 45nm test GPU, produced by ATI, codenamed RNV717 and clocked at 2000MHz. No, really, they're just marked as 'G70' to put you off the scent.
No, it's an A2 G70 cores that hasn't quite made the grade as a 7800 GT or GTX on PCI Express. Paired with Nvidia's BR2/HSI PCI Express-to-AGP bridge chip, the G70 gets to ride a very short PCI-E bus to school with AGP 8X compatibility on the far side.
GeForce 7800 GS AGP boards are specified with 256MB of GDDR 3 at 600MHz (1.2GHz effective), endowing the 375MHz GPU with gobs of memory bandwidth with which to get the job done. Texturing rate on mainstream GeForce 7-series products is limited by memory bandwidth, so the apparent excess shouldn't go to waste. There are 16 pixel pipelines.
Since it's powered by a G70, the GeForce 7800 GS AGP gets some excellent additions to image quality and potential performance per clock (and Watt), compared to 6-series products like 6800 GS.
The G70 allows for transparency anti-aliasing, a rendering ability that allows the GPU to essentially push alpha textures through the antialiasing hardware for extra Z or colour samples outside of what they'd usually get, at a sub-pixel level. The resulting extra data allows the chip to draw alpha textures at higher IQ than normal. The G70's texture cache gets tweaked for better access performance when filtering, so the hit for applying IQ enhancing texture filtering is less than 6-series hardware.
The G70 is also better at issuing scalar instructions in its vertex units - of which there are six in the 7800 GS AGP - which I guess to be down to auto-vectorisation tweaks. Triangle setup is appreciably quicker than 6-series per clock and it's well known that the fragment hardware in the G70 will do an NV35-esque double MADD issue per cycle, 4D wide.
In easy to understand terms, the G70 takes a poop on NV40, clock-for-clock, especially when it comes to pixel shading. So even moderately clocked there's a lot to be said for it.
Plenty of memory bandwidth keeps it all fed and the IQ improvements like gamma-correct antialiasing are all very welcome.
Unlocking the masked units and clocking it a bit are left as exercises for the reader. Fancy a peek at a retail board? BFG stood up to the plate, supplying us with its GeForce 7800 GS OC - and it's quite unlike any other GeForce 6 or 7-series board yet produced.