when u make the lamps just with a texture it looks very strange from some pserpectives. when u use a texture where u see the lamps from above the car will look strange from the front for example. u can surely spend more polies depends on what u are doing with it.

gta san andreas cars have around 3000-5000 polies
any new racing game for ps2 or something has around 20000-100000
just an advice