Multiplayer race game

hi, im starting to develop a multiplayer race game.
i think im going to develop it with, FLASH / FMS / PHP / MYSQL

i want to know if someone have develop something similar, to give me some advises.

The idea is to race 20 cars a same time (that what the client want) , i think i will have a view from the top of the track, because develop a 3d view from each car , i think will be more difficult.

I want to know if someone has an idea on how to develop something like this.