Well, I'm going to say go with at the very least a 15 inch LCD screen. That should be good for most general purpose gaming. And I guess my 500 dollar price range is about off, but I think the point remains that this will be cheaper than a laptop.
And you're going to sell these? Just curius, but how are you going to sell them? Are you going to open up a website or will you sell these to friends and the like?
EDIT: Whoops, dead
