OK. I just found a couple of benchmarks (in both, 16 & 32bit) of the Radeon 8500 and the GeForce 3 Ti500/Ti200,
I noticed the difference between 16bit and 32bit framerates are very small in both ATI and NVIDIA.
So I guess its an old thing, that when you run a card in 16bit, it runs much much faster (like the old good GeForce 2 GTS)
Thanks for your help AMD_MAN.