Felidae, I have the same phone as you. I found the best way to get your own ringtones on there is to download the ones you like in Midi format, and use the PSM converter which converts them to Samsung/Yamaha's ringtone format.
Next you'll need to upload them to WAP enabled server (I use tagtag.com) where you'll browse with your phone and download the ringtones.
Try searching howardforums.com for this type of information as that is an excellent resource. Let me know if you have any further questions. I haven't done this in a while (I've filled up my phone and stopped putting anything else on it now)