OK.. this is for my 8800, so I don't know if it will work for anything else......
1. Open your address book, and scroll to the contact that you want to have the custom ring tone.
2. Menu click on it and scroll to "Add Custom Phone Tune" (it's towards the bottom"
3. Choose the tone you want (preloaded or device memory were my options)
4. Save
Click your phonebook icon, select contact you wished to have special alerts/ringers on, click trackwheel to select "Add Custom Phone Tune" and select ringer or tunes you want to attach....