You can indeed play in single chip mode (the option is in 3dfx tools) but the chip will only access the 32MB it's connected to.

If you want to play jk2 with "very high" textures, either use 16bits textures (look less good) or compressed textures (I thought the option was in the game, if not try r_ext_compressed_textures 1 in the console).
But loading takes ages with texture compression enabled , the CPU converting all textures to S3TC (it's something like 2 minutes on a recent PC (say 2ghz) and 5 minutes on an older one (say 700mhz)).

BTW for compressed texture support you'll need to use either Wicked GL or 3dfx ICD v1.0.0.0761,  not MesaFX

I must correct myself, the game runs well after some tweaks on wickedGL control dialog, I've set it to compress any suited texture, and a 32mb compressed textures cache and the FPS is now acceptable.
Dunno what did the trick of the 2 things ive changed, the loading time has been like 5 minutes now...

On the wickedGL control program i see now:

Compression stats (last run application)
Quality control accept/reject ratio: 100%/0% Cache hit/miss ratio: 4%/96%
Average compression error: 4.6%  Total size of compressed textures: 51MB

51MB Oh my god!