Can’t I just get a cheap electret microphone, connect it to a pre-amp (for example, this one…http://www.sparkfun.com/products/8872) and then wire the output of that pre-amp to one of the analogue input pins on an arduino and then use the analogueRead() function to decide how noisy it is?
Won’t this do what I need? Or is it more complicated than that?
It's a little more complicated then that. After the pre-amp you still have a audio AC voltage and the arduino analog input pins require that the voltage remain in a 0-5vdc range, no negative voltages allowed. What you must have after the preamp is a diode rectifier/filter circuit that will then give you a DC averaging value representitive of the AC signal. The other problem is that such a circuit will be basically a linear responding circuit and sound levels in the real world cover a hugh amplitude range and generally require a logrithmic amp to scale the range for useful measurement.
So if your after a somewhat accurate 'loudness' measurement then you have further research to do. If you just want a circuit that gives a general "this sound is louder then that sound" kind of hardware/software function then just add a rectifier/filter and make sure the signal cannot go higher then 5vdc.