Before you even begin looking for an MP3 player, a good place to start would be to begin transferring your CDs onto your computer. Just stick one in the CD-ROM drive and see what happens. If you are using a PC with a Windows operating system, you probably already have their Media Player software installed, and you should be prompted with a list of things to do (listen to the CD tracks, rip them onto your computer, etc.) If you have over 300 CDs, it will certainly take a while to get them all on your computer's hard drive, but I found that if you spend a lot of time at your computer anyway, a bit of multi-tasking (doing busy work, while occasionally changing CDs while they rip) makes the process a bit faster.

Once you do choose an MP3 player, you can simply transfer the already converted MP3 files right from your computer, and there are numerous ways to connect the players to your car and home stereo, and build some custom playlists, that make your CD collection gather dust. I hope this helped. Good luck and have fun.