Don't use a full song as a ringtone. First of all, the phone is only ringing for what, 20-23 seconds, so you're wasting a lot of valuable space by having the full song loaded.
Download a program called Audacity - freely available for Windows and other platforms.
Once downloaded, trim your mp3 down to 25-30 seconds. You should be left with a file that's only about 300k, at the most when using high sample rates, much less at lower sample rates.
Load that file to your phone and use that as your ringtone.
New Blackberry user