You can use a program called PPC-6700 Hacks. It has an option for you to change the directory for your ringtones. However, all system sounds for notifications etc must be in \windows to show up in all apps.
Its a standard cab so... I'm hoping you know what to do!
The devs of the program no longer 'exist' so if there is a problem with me posting it, let me know admins
DO NOT DO THIS IF YOU HAVE THE STOCK PPC6700 ROM!
It's a problem if you do because incoming calls wake up the phone, and if you have a stock ROM (all stock ppc6700 rom have the DSOD problem) alot of times the memory card will not properly boot, which causes the system to hang in the off position. You wont know that it is until you try to turn it on.