Serch in goolgle for driver 2 ps1 version pictures, and you will see that driver graphics is like gta3 with smaller pixelated textures. Cars shadows look the same and player shadow for driver is even better. In gta 3 there are shadows from trees and buildings, but they are not realtime shadows, so PS1 would handle it. 2dfx effects look same, so lights are not problem too. Map would be visible in a bit smaller distance and draw distace would be smaller and LODs even more primative. Since for PS1 game ROMs have memory limitations, gta3 audio would have lover bit rate, because in gta3 audio is one of biggest folders, and as I wrote it before- textures would be more compresed and smaller, so that would decrease memory of gta3 ps1 ROM. In that way PS1 could handle gta3.
PS. sorry if there are some mistakes in text, I don't speak english wery well.