Add on kits are readily available. They are basically a plastic or glass overlay for the screen and generally use either a USB or a serial connection. Check out

http://www.digitalww.com/121TSK.htm

http://www.magictouch.com/addon.html
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...