You can always use the analog pins for digital writing.


 - `digitalRead()` works on all pins. It will just round the analog value received and present it to you. If `analogRead(A0)` is greater than or equal to 512, `digitalRead(A0)` will be 1, else 0.
 - `digitalWrite()` works on all pins, with allowed parameter 0 or 1. `digitalWrite(A0,0)` is the same as `analogWrite(A0,0)`, and `digitalWrite(A0,1)` is the same as `analogWrite(A0,255)`
 - `analogRead()` works only on analog pins. It can take any value between 0 and 1023.
 - `analogWrite()` works on all analog pins and all digital [PWM][1] pins. You can supply it any value between 0 and 255.

The analog pins let you read/write analog values - basically, instead of giving out a voltage of 0 or 5 (as with digital), they can give a range of voltages between 0 and 5 (both as input and output). Note that the voltage during analog output is only the observed voltage with a multimeter. In reality, the analog pins send pulses  of 0V and 5V signals to get an output that "looks" analog (this is PWM).

Regarding the number of pins: keep in mind that the PWM pins can be used for analog output. If you run out of pins, you can use [multiplexing][2] to make more. It is not necessary to get another Arduino.



  [1]: http://arduino.cc/en/Tutorial/PWM
  [2]: http://arduino.stackexchange.com/questions/4/how-do-i-extend-the-number-of-analog-input-pins-available-to-me