trust a power supply?

I got a power supply which is about 12V. I'm wondering does it actually fluctuate from this rating? I know that the arduino itself can take a bit more than 12V but I was wondering if it would harm stuff attached to the arduino.

There's an onboard regulator which converts the 12v to 5v

So less voltage (7v ideally) is better, what's the current rating? 12v how many milliamps?

The regulator can take more than 12V as long as you don't draw too much current through it.
Otherwise it may heat up and shut down.

It is 2amps. The attachment is recommended to have 2amps for some reason.

http://wiki.epalsite.com/index.php?title=SIM900_Quad-Band_GPRS_shield_with_Micro_SD_card_slot

I have a difficult time finding PSU that come with current that big. All I can get is either 12V 2amp or a 6V 2.1amp. Would it be better to order a lower voltage with the proper current on ebay? Or is the current recommendation bad?

Actually, what you want is a fully regulated, presumably switchmode, power supply for 5V, 2A, which you then connect directly to the Vcc on the Arduino and your shield.

The shield apparently has its own regulator, presumably actually operating internally at 3.3V, and it specifies it will run from 5V, so a well regulated 5V will do just fine. You need the 2A for RF transmission bursts.

Paul__B:
Actually, what you want is a fully regulated, presumably switchmode, power supply for 5V, 2A, which you then connect directly to the Vcc on the Arduino and your shield.

The shield apparently has its own regulator, presumably actually operating internally at 3.3V, and it specifies it will run from 5V, so a well regulated 5V will do just fine. You need the 2A for RF transmission bursts.

I'm a little confused. The VCC is a pin right? Isn't it better to connect through the power port at least initially? I know that the shield can operate at 5V but the arduino itself needs around 7V. I can't connect a PS to both because that would damage it I have to choose only one to connect to. So I was wondering what sort of voltage would be best. I have a 12V 2amp and a 6v 2.1amp or I can order something different off ebay. Also how do you tell if a ps has switchmode and is fully regulated?

The microcontroller itself requires 5V.
It's the voltage regulator than needs higher voltage (7V) to generate 5V.

If you have a proper 5V supply, no harm will come to the chip if you connect it directly.

Most Arduinos include a linear regulator which regulates the voltage from the "Vin" terminal (which is much the same as the "barrel" connector if that is present) to the 5V to operate the chip. The board can be powered by this input at 7V minimum or with 5V via the USB connection or with 5V via Vcc. And it happens that if "Vcc" is not itself being used to input power at 5V, it can provide 5V from whichever source - USB or regulator - for other components within reasonable limits (about 200 mA). In any case the "Arduino" is running at 5V - or a lower voltage if that is supplied through the Vcc pin and the clock speed is compatible with the lower voltage.

7V is merely the minimum specification for reliable (accurately regulated) 5V from the internal regulator. If you provide properly regulated 5V from another source into Vcc, that will work perfectly well and if that 5V supply is adequately capable, it can also operate other devices - such as your shield (which clearly contains its own regulator to drop to its actual working voltage of 3.3V) - as well.

A switchmode (or "switching") power supply is usually specified as such and is almost always regulated to some degree (especially if it has an input voltage specified as 100 to 240V AC). Most advertised as 5 volts will be this though there may well be variations in quality (and reliability).