PC fan speed control
Question #1: Which power supply should I use?


That depends on the rating of the fan you're going to use. If it needs more than 300mA then use the PSU that can push 2A. Usually there's a sticker on the center of the fan that gives the rating. If there's no sticker, using PS1 is okay, the fan will only take as much current as it needs.


  Question #2: What size potentiometer should I select given the power supply such that I can control the speed of the PC fan?


Usually you find out the resistance of the fan and use a rheostat in multiples of that value. (PC fans usually start at 50ohms). If you can't find that out though I would suggest using a variable voltage regulator like the LM317 to vary the output voltage to the fan.