All modern sound cards should support 8-bit and 16-bit samples and sample rates of 22050 Hz and 44100 Hz. Some sound cards will support a greater range of recording rates which can be lower and higher than these values. The familiar format of CD audio uses a sampling rate of 44100 Hz and a bit-depth of 16-bits. These values are more than adequate to represent almost all real-world signals for listening by humans - and also, conveniently, are fine for Amstrad tapes, too! In fact, in theory, because the standard Amstrad tape routines have a maximal frequency of 2500 Hz, settings as low as 8000 Hz and 8 bits would probably be fine. However, you will probably want to use higher settings, just in case and/or to keep in line with more common formats such as CD audio, especially if you intend to archive your recordings.
The hardware tape data separator inside the CPC only extract 1-bit of information out of the sound signal that comes in. So, using 16-bit instead of 8-bit samples provides no gain at all.
For samp2cdt, you should save the file as "PCM" ("Pulse Code Modulation"). This is a uncompressed, unencoded storage representation. Each sample is a single measurement of the amplitude of the sound taken at a measurement point in time. Other representations such as "ADPCM" ("Amplitude Delta Pulse Code Modulation"), encode or compress the data to reduce the size of the audio file. Samp2cdt can't understand these representations, so please use "PCM" only. Another reason to avoid "ADPCM" is that it is a lossy compression format. "PCM" is lossless, so it's better suited for data storage.
===Illustrations and explanations of digital audio===