You have two options. The first is to go the riskier, cheaper way, and buy a bare drive and install it yourself. If you want to do this, you can get
this Panasonic drive and install it in your PowerBook. You will also have to use Patchburn II to get full drive compatibility. For more information about all that, go to
www.xlr8yourmac.com and look at the drive compatibility database. It's pretty inexpensive and you should definitely consider installing it yourself.
Then there's the safe but MUCH more expensive way to go. You can buy an upgrade service from MCE Tech
here and they'll replace your old drive with a new DVD burner. The problem is that they charge a whopping $250!
Either way should work fine, it's just a matter of whether you think it's worth the extra $110 to have someone else do the installation for you. Also remember that if you replace the drive yourself, you can resell your old Combo drive, possibly to someone with a DVD-ROM equipped 400-667 MHz TiBook.