If you're making a portable you probably need something to watch it on. (Unless you want to guess what's happening in the game, but I wouldn't advise that) Anyway, this forum is your "Hacking a pocket TV/screen" one-stop solution. Share your experiences and knowledge here.
At least 6V is better, since the screen looks kinda dim at 5V. Ben uses some screens that (I think) can run on 6V. Tell us what you are planning on and we might be able to help you more.