I'm curious about why you want to build a forearm computer?
Kurzweil -- who's been pretty on the money for 35 years with his technological predictions -- shows that it is likely to be placed closer to the ears. Like a hearing aid today or just the bluetooth attachment. I think he's right. We've got the first toe-dip in the waters of Augmented Reality with the Google Glass project. By 2020, the AR technology will incorporate computers, internet, instant messaging, phone service and personal assistant service. It will recognize your tone of voice, answer questions before you have them and offer information on everything you're looking at.
You'll need it. Because, like a particle approaching a quantum singularity speeds up, as we approach the "technological singulairty", the increase of the spead of advancement will, itself, be advancing. It will reach a pace where the advancement of technology will be too fast for us to keep up with comfortably, thus AR -- medically inserted or worn on the face -- will aleviate some of the discomfort with the complexity of our environment.
~String