hmm files going upto 200mb??? how does your D70 make those hahaha, ok I know you mean files with layers etc...
well before I got my G5 I worked on a TiPowerbook G4 400 with 1GB of ram and it wasn't fast but worked ok... so an iBook should be fine... there is allways something faster, but not portable
one thing: I've heared that the colour/luminosity of the iBooks is not so good, PowerBooks have better screens... so perfect calibration might be an issue... but buy the EyeOne calibrator its quite good even for 'bad' screens, but you should know: don't ever trust a screen blindly... measure your colours to be sure about things for instance my screens shows 'details' in white parts that cannot be printed so sometimes I look to my screen and think: damn whats that shade doing there, but its invisible to print (and I don't see it on other screens, so its only here ;-)
and as you know photoshop eat ram alive, get as much as you can get...
there is one photoshop theory: take the size of you biggest file you commonly use wich is 200mb in your case? x3 = the minimum amount of ram needed to work nicely with Photoshop
set your memory settings in photoshop to 75% (leave some space for other things...)
and get Photoshop CS2 because file browser is now bridge a standalone program thus not slowing down photoshop as it used to do ;-)
but there is as I said only one rule: get enough ram
a friend has 3GB ram in his G5 and when I come over with my big scans (1,2GB) they open like it were small jpegs

mine has to start thinking a little bit then ;-)