Actually, you can do it native with the Treo 650. 1) Go into the phone app. 2) Go to the menu item: Edit Favorites Pages. 3) Click on a blank box, and change "Type" to SpeedDial. 4) Add the person's name (use the Lookup botton) 5) select the Ringtone you want from that screen (bottom pull-down)and hit OK
By default you can assign different pictures to contacts. You can at least buy and use Ringo to assign ring tones. Although Ringo is very nice (also includes a lot of neat music stuff) you have to create your own Ringo database as it's not incorporated with the contacts menu. For this reason, I am still searching for a simple utility which will just add this function to the contacts.