zigzog:
I'm using a voltaje regulator(http://www.analog.com/static/imported-files/data_sheets/ADP3338.pdf). It outputs nice 3.3v to all the sensors I'm using. Is it feasible to use the output of that regulator to power the card reader? Or better go with the 3.3v Arduino?
It would be easier to run the Arduino at 3.3v, especially if you have various 3.3V sensors. In theory you should not run the ATmega328P at 16 MHz on 3.3V. That is why they ship the "3.3V" version with an 8 MHz crystal. It will almost always work at 16 MHz on 3.3V..