What kind of tool is used to record a voltage over time to a PC file like a waveform?
This is called data logging. Your application is a bit out of the ordinary only because of the large data files generated. Measurement Computing will sell you a 12-bit, 500 kHz digitizer for under 100 bucks, and the free software wil theoretically do what you need. However, the data sheet for the software warns that files over 1 M "may suffer performance issues". Other software packages are available, and I'm sure one of them will do the job.