Most touchscreens have or are moving to PoE (Power over Ethernet). If you use an in-wall iPod / iPad dock, options exist to power over cat5 cable. So cat6 is a sensible option. Might not hurt to run a 2-conductor (16/2) either.
Keep in mind that (for the most part) the touchscreen is just the user-facing part of a control system. The touchpanel will store GUI graphics and communicate button-presses and actions back to a head-end processing unit; maybe it can also receive video and other feedback from that control system processor. But the actual communication to and from av equipment, lights, shades, & thermostats is managed by a control system processor. The touchscreen is not the central hub of the system.