well, hi-poly are for close up scenes, low-poly's are for normal and far shots.

if i were you, i'll model them in low poly's and if it's a close-up shot, just add a few details. you can always switch the details from high to low using the Multi-Res modifier if you're using 3ds max.

hi-poly's wont hurt your RAM too much, cuz if you're doing a really big scene, just do some greenscreening with it later in video editing softwares such as after effect.