I have the same itunes/iphoto problems, with a 160gb and 560gb respectively. I solved this through setting up 2 external drives, and storing stuff there.
- for itunes stuff, I just transfer the stuff (videos/tv shows mainly) to the internal drive locally when it's needed. Plays just fine using quicktime; itunes itself is not needed.
- for iphoto, I leave it all on the external, and use the photostream feature for stuff you want portable, though it's a bit hit and miss.
- 2nd drive is for time machine.
Not an ideal setup I admit, but at these sizes of both libraries not many laptops (win or OSX) can take both completely.
No, the real problem is imovie. It's a storage bitch if you plan to be portable.