If you're ripping the songs yourself from a lossless format to MP3, the program will usually let you trade off between compression and quality. I suggest you make copies of the same song at numerous decreasing levels of quality and listen to them till you find the point at which you can just begin to hear the loss of quality, then just rip a level or two better than that. Everyone's ears are different.
Yes, thanks. I have tried to find the best balance between sound quality and file size.
But I will still keep the old CDs, as long as I have room.