sense movement of muscles via electronic signals
First, fingers don't have muscles that you can measure potentials from.  All the muscles that move fingers are in the forearm, with tendons extending into the hand to move fingers.   The anatomy is complex.  While a pro can sort things out, I wouldn't assume that you would be able to pull out enough info to differentiate between fingers.  (The thumb is a different story).

Assuming you will eventually be measuring muscle activity, which is an electromyogram, or EMG,  this is a high frequency AC signal.  It can be teased into a very rough estimate of muscle tension by rectifying it and then low pass filtering it at about 10 to 15 Hz.   You can do this in software, but you can sample much slower if you do it in hardware. It will be difficult to calibrate between different muscles. I'm not sure if the EEG shield people are talking about will let you sample fast enough for raw EMG before rectification and filtering.

I'd go another route, like a glove with flexiforce sensors.