Hello!
You can definitely upgrade your machine to the 1.67 GHz (Model A1106). Any of the logic boards on the [http://www.ifixit.com/Mac-Parts/PowerBook-G4-Aluminum-15%22-Model-A1106#0-Logic+Boards|A1106 page] are compatible. However the 1.67 GHz (Model A1138) has a different layout and is not compatible.
You do not need to match VRAM (it affects video performance, but is not machine-specific). Just for information's sake, the 1.5 GHz (Model A1106) has 64 MB VRAM onboard.
I hope this helps!