You can re-encode MP3's you already have on the PC with smaller bitrates so they take up less space on disk. 128kbps is CD quality. So not much point in ripping them higher you won't physically be able to tell the difference. Only if your playing live in a huge club or something. Variable Bitrate is OK, but most Dj mixing applications don't reccomend you use it.
If a CD is already mixed and you don't want to be able to hear the gaps when you play it back on your MP3 player use a program called "Exact Audio Copy" it has an option to encode a "Range" this will copy the CD as it should be played as one big MP3 instead of lots of individual tracks with gaps.