Space isn’t really a problem though, except for the internal memory on the Clip Sport, as when using 256kbps or higher bitrate one probably won’t get near the 2,000 file limit. I bought Some Sandisk 32GB class 4 micro SDHC cards recently for around $13 each. As for using variable bitrate mp3 files, I haven’t tested it, however I have a feeling that they may consume more battery power than constant bitrate files around the same sound quality. I also haven’t tested the claim that 192kbps average variable bitrate mp3 files sound as good as 256kbps constant bitrate ones. When I did tests years ago, I had trouble distinguishing between 256kbps mp3 files and 320 kbps ones, so I chose 256kbps. If I was ripping CDs now though, I would use 320kbps since storage space is very cheap now. Even now,not all devices support variable bitrate playback. Years ago when I ripped many CDs I decided to avoid variable bitrate since not all devices support variable bitrate files, and many that did then had plenty of glitches in playing them. I still wonder about battery life differences between playback of constant bitrate and variable bitrate files.
As for hearing differences between files, see how well you do on this test.