A little contribution -- these two are from my (well, now sold to a coworker) Nokia E50 -- Ring_Ring.aac sounds great on my Pearl, it's very business friendly.

http://rivviepop.php0h.com/bb/tone/Ring_Ring.aac
http://rivviepop.php0h.com/bb/tone/Notify.aac

PS: when the media player launches and plays the tone, if you select Save it will save the file with a .mp3 extension; it's actually still a AAC file, so you should rename it. Small bug I guess.