Do not power the Arduino board from the USB in the long run, especially if you are interfacing external signals and hardware.
Use a 9V to 12V regulated Wall-Wart as suggested, and configure the Arduino for external power. The 5V regulator on the Arduino board will take care of the 5V power. Tie the OV side of the Wall-Wart to Gnd on the Arduino.
Power the analog input amplifiers (LM324) using the 12V and OV from the Wall-Wart. The LM324's output pin can pull from near 0V to Vcc-1.5V, so if you power them from 5V, your analog signal can only go from near 0V to 3.5V. If you power them from 12V, then the LM324's output pin can swing over the entire Arduino input range of 0V to 5V.