Exactly what I was trying to do. Here is what I found.
Plug Sensation into PC using Micro USB lead. On phone say that you want to use it as a disk drive. On PC, create a folder in the root of your phone's drive called "notications". Copy your mp3 sound into that folder.
Disconnect phone from PC. On phone, click on messages. From main screen there, got menu->settings->notifications. Under notification sound, your mp3 should appear in the list of available sounds.
The above worked for me, but I had a slight problem in that instead of the track name showing, the album name appeared!!! Still works though.
If my instructions aren't completely clear, try googling: htc sensation mp3 text notification
As this is my first post, I can't include the link, but the above should get you there okay.
Now I need to find out how to do the same for email notifcations!!!