Ummm...
*scratches head*
Maybe I have missed something here. The PowerBook 12" will do a lot more than 1024x768 on an external monitor, you are thinking about the 12" iBook which won't without the screen spanning hack.
64MB of Video RAM is plenty enough. I regularly edit huge files, say 16x12" files at 300dpi on an external screen, have I seen it stutter, not at all.
Bigger VRAM is needed for 3D games more than anything, but 2D editing, 64MB is fine and has yet to cripple me. Sure you can't run a 30" display but if you're like me, I don't have £2k in the bank burning a hole so it's not a problem.
12" does not have FW800, but I have backed up my whole hard disk using FW400 and it hardly can be described as taking an age, plus the 400Mbit/800Mbit speeds are quoting the peak transfer rate rather than the mean transfer rate.
The 12" PowerBook is fabulous, and just as worthy as the 15 and 17" models; the difference lays more in the built in screen size than anything.
The 167MHz speed difference in the 12 and 15/17" models will be a minor issue and barely noticeable for what you will be doing.
http://www.lilserenity.com shows off in the portfolio some of my work completed with a 12" PowerBook G4; it is very capable for doing graphics work and I still don't know of another ultraportable with a dedicated GPU and 64MB of dedicated VRAM with an optical drive.
Vicky