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.
1) 7" screens (and 16:9 in general) often stretch the video. Sometimes, like the above picture, it cuts off the top and bottom of the image. Games are designed for a 4:3 ratio.
2) Often times when you find a screen with a good resolution for a cheap price, there's something wrong with it. It could be large circuitry, colour bleed, extremely low refresh rate, poor colours to begin with, etc.
Looking for a trans-reflective screen for my University's Solar Car, we stumbled across a site that had an amazing-looking screen. Trans-reflective, 640 x 480 resolution 5" screen. The catch: 6-bit colour.
There's more than just size and resolution to consider when buying a screen.