The jumpy polygons are because the PSX's GPU only accepts vertex positions in screen coordinates, meaning that vertices always snap to hard integer values. There is no sub-pixel accuracy. That's not a limitation caused by the PSX's lack of an FPU though. A large part of the render pipeline and standard libraries are built around the use of fixed point math, i.e. using integers to simulate decimal values. The GTE co-processor which handles 3D math makes use of standardized fixed-point types. So the PSX would have been capable of calculating sub-pixel coordinate values, it's just that the communication between CPU and GPU was designed to use only integer screen coordinate values.