The arduino onboard regulator can handle up to 1A if you plug your walwart into the dc jack. If you want to bypass the onboard regulator you can get a female jack for your walwart and connect the ground and +5V to the GND and +5v pins on the arduino. It would be connected to the OUTPUT of the onboard regulator but it wouldn't matter because the onboard regulator would have nothing connected to it's INPUT because it's INPUT is the Vin pin. It would essentially be like it wasn't there since no current would be flowing through it with nothing connected to the Vin. The USB +5V is a DIFFERENT input so if you connected your USB to the arduino WHILE you had your walwart connected to the +5V & GND then you would have TWO +5V power supply OUTPUTS connected TOGETHER. THAT WOULD BE BAD. If you think you can remember to disconnect your walwart BEFORE plugging in your USB to upload sketches you would be ok.But trust me. Some day ,some time , you will be tired or distracted and sooner or later you will make that mistake and all life as you know it will cease to exist. (or maybe you'll just fry the USB in your computer. Have fun fixing that.