What about changing the text notification so it is softer once you are in the application. On my iPhone, there was a standard notification sound when you first got a text message. One you opened the text app and began texting back and forth, the notification became a much softer swishing sound. It's kinda loud on the Incredible when every incoming message has the same initial sound....even when you are using the app.
Add a folder called Notifications to your SD card. I used phonezoo to download numerous ringtones to my computer. Put what you want for notifications in that folder and then go and choose the one you want when you change the notification sound.
If you want unique ringtones for your contacts add a folder named ringtones to the SD card and put yor downloaded ringtones in there. Now when you go to assign a unique ringtone to a contact you can choose from those.