I dont know if this helps your situation any, but ill put it out there anyways...
I wanted to try and keep my 5 year old 12" Powerbook alive with maxed out ram because I didn't want to make the Intel transition and upgrade my software $$$ but after a week of the new ram, I was still annoyed about the sluggish performance. So long story short, I went out and made the transition to the maxed out 15" Unibody MBP.
As I powered up my new computer, and set it next to my Powerbook I laughed out loud to how dim my Powerbook was in comparison. I can't look at it anymore and cannot wait to hand it off to my parents.
The new LED screen is "blinding" in comparison to the old matte lcd on my powerbook, I would never go back. It is simply incredible. Now all I gotta do is put down a few paychecks on CS4 and FCP and Ill be good to go.
IMO, putting more money into a 12" Powerbook wouldn't be that great of an investment unless you have no need for processor power or a video card that can run anything decent.