MarkT:
Too much current won't hurt, too much voltage will. A "12V" wall-wart might give 20V without load if its unregulated - its important to know if the supply is regulated (much prefered).The higher the current at 5V you want to use with your circuit the more important it is to keep the power supply low voltage (the regulator on board turns excess voltage into heat proportional to total current). A good quality 5V regulated supply can go directly to the 5V pin, a regulated supply from 7V up goes to Vin and uses the on-board regulator.
Thank you.but if use a regulated Wall Adapter 12Vdc (1A) with the (uno+ethernet shield) 24/7 will it harm any thing or i should get a lower voltage due to the working period.