I use the same software you're using, I do a lot of web design, and have recently moved from an iBook 14" to a MacBook, and have been very happy with the transition.
I wanted more desktop real estate on my laptop, but I couldn't afford a MacBook Pro, and didn't want the extra size and weight in a 15" PowerBook.
The widescreen res (1280x800) of the MacBook is just perfect for me, giving me the extra space in Photoshop and Dreamweaver 8, but at 13.3" wide, a little over an inch thick and slightly less tall than the 12" iBooks and Powerbooks, it's a slim and powerful companion for me on the move.
The PowerBook beats the iBook handsdown for image/design work, simply because of the added desktop space you'll get over the iBook's 1024x768 max res (Powerbook 15" - 1440 x 960), and if you don't mind the extra size and weight, it would be your best bet for the $$ and its ability to handle future apps.
Rosetta and Virtualization
On an Intel Mac, 10.4.8 Runs PPC applications like Photoshop and Dreamweaver beautifully. I say 10.4.8 because the Intel version of the update included significant Rosetta performance increases. According to a recent MacWorld report
, up to 30% in some cases. Previously there was much more noticable 'drag' in the performance of larger applications that required Rosetta.
The fans rev up in the MacBook compared to most other tasks simply because of the extra processing required to translate the PPC apps using Rosetta, but you won't really see a performance hit on the latest Intel hardware - actually you might still see an increase in speed, as the dual core processing power, coupled with the faster ram in the latest Intel Macs seem to make up the difference, and then some.
One of my major considerations was being able to run Windows virtually alongside OS X at around 90% of it's native speed through Parallels. This allows me to view my web design on Windows and Linux web browsers, giving me the advantage ot found elsewhere to view a given website design in over a dozen browsers simultaneously
With 2GB of ram in my 2GHz MacBook, I can run parallels with WIndows XP and Linux, Dreamweaver 8 with 10-12 pages open, Photoshop with multiple images, and Safari, Firefox, Shiira, and Camino open, not to mention Mail, iChat, and Cyberduck.
The later rev. 1.67GHz 15" Powerbook will come close to doing just the same, and if you don't don't need to run another operation system it'll possibly a smarter bet as it will handle the PPC apps you need with ease, and give you some features you won't get with the MacBook.
As for obsolete, far from it. These machines (iBooks & Powerbooks) are going to remain useful, even desirable over the MacBooks for awhile. Programmers aren't about to abandon the PPC crowd for years to come, nor will Apple.