it really depends
the average polycount on screen in ut2004 is 150.000 (models+terrain)
the average polycount for models was something like 2000-3000
the average polycount for terrain goes from 100000 to 800000
A pair of years ago J. Carmak "dictated"
the tendency that he didnt needed more polycount but more and more powerful processingto those polys, so we will notsee (and arent seeing) spectacular polygon # advances but increases on the fancyness of the shaders.
(the "# poly muscle" of a geforce3ti to a geforcefx5200 wasnt that much indeed)
From my experience a radeon 9800 can stand 2000000 polys at 25fps in directx just by brute force if you do nothing fancy with them. (ie 1000000 billboards, no lights just ambient, showing a 256x512 texture). If you start adding shader passes and lights shadows and things or try to do something else like running game logic /whatever the performance decreases exponentially...
but im like a thousand miles from being a programming or game modeler guru anyways...