The S3 Virge is a first-generation 3d card that's not even capable of proper blending or fog, made from back when 3d acceleration was a very new concept in the consumer PC market. The Geforce6 is built upon years of technology advancements upon many more years of API maturity. They just don't compare.
The best 3d hardware that's emulated in PCem currently (as of v13.1) is the 3dfx Voodoo2, and even that doesn't work with everything mainly because of its design as a strictly secondary video device.
Emulating the Geforce6 would need nVidia to open up their docs and info on their cards to the public (Which are often confidental material and this is unlikely to happen) - or probably decades worth of hobbyist reverse engineering.
The reason why 3dfx emulation's everywhere is that 3dfx opened up nearly around their closure in 2000 (and there was also a time when 3dfx support was a priority in 96-99, so emulating 3dfx, like it or not, is the most logical thing to do for any gaming-centric PC emulator)