Just take the "ringers" you would like (songs from your computer) and add them to folders on your SD card structured as follows:
MEDIA
-ringtones
-notifications
-alarms
Add the files to the folder you would use them as...If I want happy birthday as my text or email ringtone I would add it to notifications...and if I want to wake up to rockin robin I would add it to the alarms folder
Then when you go into notifications (text ringer) in menu settings...your newly added files will show up as choices.
To change the "ringtone" in email go to your email account and press Menu, More, Notification Settings