Wirelessly posted (BlackBerry8700/4.1.0 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/102 UP.Link/6.3.0.0.0)

I don't know about MP3 ringtones yet, haven't downloaded any yet, but to add a custom tune to a specific contact:

Edit contact,
Menu,
Add costom phone tune,
Menu,
Change option.

Don't' forget to save!

Hope this helps.
Rcbjr