Changing the default SMS notification

Morning.   First post.

A week ago, I bought the HTC Desire.  I have done, I think, all that the various posts say in order to change from the default or provided SMS notifications.   I have established the various folders on the SD card, and copied my preferred MP3 tunes into a number of them.  I then choose the tone from the list, but I still get the default.

One thing that may be influencing my lack of success is that I have not trimmed the MP3 tracks to 30 seconds.   Could that be significant?

Are there any successful step by step processes, on this site or elsewhere, showing how to change the SMS notification?

Many thanks   

I know, kinda necropost, but just a reminder for who wants to change: i used an app called "ringdroid" available in the market, and it worked like a charm