im buying a new iMac next week!!!
ok, i had to edit my post because i read your first post wrong ..
i was originally going to get an iMac 20", but after endlessly searching the internet for the last 3 weeks, as well as going to the apple store and mac store (they are a chain up here in the PNW that is an authorized reseller) and comparing the 20" to the 24" running the same programs, the screen quality on the 24" is much, much better.
I asked the difference in speed between the 2.66 ghz and the 2.8 ghz and if i would notice a difference when running CS4, and the guy said i wouldn't. The biggest differences in speed and performance are going to be ram (always buy the 2GB and buy a 2x2GB stick from newegg and upgrade yourself) and the graphics card. The difference between the 256MB and the 512MB card is going to be tremendous, especially since CS4 is going to be more graphics and resource hungry more than ever before.
The 512MB card only is available on the 24" model, and it is a $180 option on the 2.8 ghz model.
I am an avid photographer as well, and the need for processing a 40-60MB file (sometimes a 32bit file as well) with several programs open at once will essential.
Also one last thing. For me, spending the extra $400ish (wireless keyboard, graphics card) over the 20" iMac is worth it because this computer will last me through school atleast. (i am going to art school for photography and web design).
good luck with your choice. i cannot wait until next friday!!!!!!