We will be covering the NES in this Instructable, but you can make a portable out of any console you want. Some are harder than others, though. Here are the "main" game systems that would be easy to make handheld:
Nintendo Entertainment System
Super Nintendo Entertainment System
These systems are slightly harder:
And these are VERY hard to portablize:
Obviously, the last list is only for people who are more comfortable with electronics and know their way around a PCB. For a first-time portablizer, I recommend the Nintendo Entertainment System. This console has tons of great games (SMB3, Kirby's Adventure, Legend of Zelda, and Mike Tyson's Punch Out, just to name a few.) and is a easy console to start with. The NES is what we'll be portablizing in this Instructable.
If you want to do a different console, then you need to look at http://forums.benheck.com/
. This is a huge community dedicated to making awesome handhelds, and it contains information on how to get power, video, etc. for your console, and how to cut down many systems to make them smaller. If you are going to do a console other than the NES - which is what we are going to work with in this Instructable - then you need to check out that site. Scroll down and find the forum for whatever system you want to do. Check the stickied topics in that section for all the information we have on that console. Also be sure to look at the Reference section
Enough chit-chat. Let's get to the first step of portablizing!