Ignoring the Mac Pro for now (reason later), I'd say go with the 20" iMac. Sure, the bigger screen of the 24" version is nice, but from what I read, I'd say you'll profit more from investing the price difference in RAM (max it out, if you can!) and an external harddrive. The external harddrive, btw., is key. You'll _want_ to do backups. (If you think "no" here, let me tell you that you'll want to _have done_ backups one day in the future...)
I think it's also important to learn to archive. If you really work with photos, you'll generate a couple of 100 MBs every week. But even if it's only 50 MB/week, you can still calculate that at _some_ point in time, _any_ internal harddrive of a MacBook or iMac or MacPro will be full. You can, of course, put the archiving off 'til later, but it'll be more of a hassle later on and take longer. If you archive a done project right away, its space is saved on the active main drive. And free space on the system volume is _key_ to a well-performing Mac.
Now for the reason against the Mac Pro: Money. A well-equipped iMac or MacBook simply gives you more performance than an under-equipped Mac Pro. Sure, if you _can_ afford to give the Mac Pro enough RAM and harddrive space, that's just dandy, but if you have to sacrifice RAM and harddrive space (or a backup solution that is not internal), it'll bite you back soon enough.
So... Why not do the following:
1.) Get an external FW/USB-2 harddrive now. Free some space on that PowerBook by backing everything up and deleting the stuff from the internal drive that you don't need at the moment. This enables you to hang on to that PowerBook just a little longer. You might want to do that, because
2.) At the end of October, Leopard is released, and the new iMacs, which should come out in early August, will then come preinstalled with Leopard. (You've just saved 129$, which might be enough for that external harddrive, depending on what you buy and where...) Now buy the iMac that fits you best. Until then, you might have put a little more money aside as well, which may or may not nudge your decision towards a different model.
The important thing, I think, is to see that having a big external harddrive will solve your initial problem _immediately_. Freeing up space on that PowerBook will also speed things up.