Create a folder titled music on your internal sdcard or external sdcard if you don't have one already. Drop all your MP3 ringtones into that folder.
Go into messaging app>menu>settings. Check notifications.
Download Ringdroid from the market. I like this way because it allows me to pick a MP3 ringtone and crop just the section of the MP3 I want as my notification or ringtone sound. If you go the Ringdroid route, you will be given an option to use the MP3 ringtone of your choosing as a notification or ringtone, crop the section you want to listen to, given option to use as default ringtone or notification, or set certain ringtone MP3's to different contacts. It is extremely customizable.