I have just completed a standalone Polargraph ( http://www.polargraph.co.uk )
It is a Arduino Mega running two stepper motors (nema 17) a micro servo and a TFT shield that also has a micro sd reader inbuilt. It is powered by a wall wart outputting 12 v DC 1 Amp.
When the servo moves the screen cuts out. HOWEVER when I have USB power as well it's all sweet. Do I need a wall wart at 1.5 Amps. or 2 amps?