Get Rings Extended
I actually own a Captivate, and the only way I was able to get around the limit was to install the free Rings Extended app. I had already put my full-length ringtones in the /media/audio/ringtones folder on the phone's internal memory, after trying to just have the ringtones folder on the root of my SD card to no avail, and it didn't work.
In order to get Rings Extended to work (it doesn't have an icon on the app list), go to Settings, Sound and Display, Voice Call Ringtone, check the set default box, and choose Rings Extended to manage ringtone selection. You can then change your phone's ringtone to something larger. To set ringtones per person, just go to your Contacts list and do it as normal. Rings Extended will pop up to allow you to choose longer ones.