If you're actually discarding the physical CDs, don't let MP3 be the only format you store them in. Apart from the fact that you should always have a second copy of everything, MP3 is a lossy format from which the original can't be recovered exactly. Store them also on hard disk or DVDs as .WAV files, or if you want to save on storage space convert the .WAV to .FLAC. (Free Lossless Audio Compression)

FLAC is free and gives you about 50% reduction in storage space - not as much compression as MP3 but still useful, and you can recover the exact .WAV file which is a replica of what was on the CD, and make fresh MP3's from those if necessary.

Incidentally some players can handle FLAC directly.

Anahata