According to id Software, this threaded renderer will shine on systems with more than two cores, so really just quad-core and octal-core systems (for now).
So far I've only tested at the settings I play at. I'll do some low res totally CPU limited tests but for now here are my results:
Opteron 165 @ 2727mhz
2gb DDR550 3-4-4-8 1t
8800GT core 700mhz memory 2000mhz 512mb
Ubuntu 7.10 x86-64 2.6.22-14-rt kernel
Nvidia 169.07 drivers
"checkpoint 1.4" demo 1600x1200 all in-game settings set to maximum 16xaf 2xaa
threaded renderer '2'
threaded renderer '0'
It might not seem like much of an increase but it improved FPS where it really counted. With the threaded renderer enabled I never see under 30fps and it drops to 30 less often.