Try putting an MP3 file on your SD card in the folder "/mnt/sdcard/media/audio/notifications/" (without quotes). Then, from your home screen, push the menu button, and tap Settings, Sound, Notification ringtone, and select the sound that you want.
I use Astro File Manager for my file management needs because it also includes a tool to backup all your applications to the SD card. There are some other programs available on the Android market that will help cut a music file to a shorter length. Try the other folder "/mnt/sdcard/media/audio/ringtones/" for ringtones.
I also wanted to mention....I turned off my e-mail notification because I found myself not check it because I would assume it would be e-mail coming through rather than my texts since I couldn't change the tone difference between the two. I would set my e-mail to be different but it would go back to what ever I set in my sounds.