What bit rate MP3 files are you using and what equipment are you currently playing them on?
PS Where did you get your MP3 files from ... are they downloads or did you rip them from your CD collection for example? If you have the original CD source you may want to re-rip to a higher bit rate or a lossless format such as FLAC.
At what bitrate have you ripped your MP3s in? Which speakers have you got? What's the source (is it in your computer)? What's your budget for a DAC? Bear in mind that your system should be revealing enough, & your MP3s at a sufficiently high bitrate, for you to appreciate improvement in audio quality, if any.