Turn off pedestrians and limit the visibility to about 1/2. Turn texture detail to High or lower (avoid High [AGP]).
There probably isn't anything major to do other that make sure you have the latest version of you Voodoo drivers, DirectX (7.0a or 7.1), and tweak the game itself.
Midtown can run pretty slow even on a fast system. I think a lot of it has to do with the engine. It's not very efficient at limiting view (other than manually turning down visibility). Even my system will lag bad if I have the visibility set very high and try to look at the entire city at once.
BP6, P3-700@770 (for now), 320meg, Prophet Geforce DDR-DVI
BP6, P3-700@770 (for now), 320mb, eVGA MX Plus (dual CRT), waiting for VIVO module