"But more importantly, is is worth installing on a 667MHz machine or is that too slow now? I use the built-in UAE to run legacy apps and games, play music and dabble with paint apps (no internet browsing, no videos)."

If you avoid web browsing or heavy sites (which are a pain on other, much faster Macs anyway), it should be ok. H264 in 360p should also be ok. The slowest Mac I was using MorphOS on was a 400 MHz Powermac G4, and the experience was quite okay. UAE is ok for ECS games, heavier stuff like AGA could be a problem.

But I can't tell you, if your Titanium Powerbook is supported by MorphOS, or if sound is supported.