You're always going to lose some quality with mp3, whether that loss is noticeable is the real issue. If you use a good quality encoder such as Lame, and set the bitrate to 160Kbps, most people won't be able to notice any difference. If you set it to something higher, like 256Kpbs, the mp3 files will be larger in size but even better quality - it's a trade-off between size and quality really.
I use 160Kbps for the few occasions that I need mp3s (I normally use iTunes/aac) and I've never really noticed any drop in quality from the original source.
A good program to use is dbPoweramp - though unless you want to pay for the lame encoder, you should download the Helix encoder from here and use that - it's very quick and still good quality.