I'm considering three different, new iMac Core-i7 models:
1. 3.4ghz (27") 2011 CTO model for $2199,
2. 2.8ghz (21.5") 2011 CTO model for $1699, or
3. 2.93ghz (27") 2010 model in stock at a major vendor for $1810 closeout, new and un-registered.
Primary use is Logic Studio and Pro Tools.
The difference between option 1 and two is neg. CPU's will give you similar performance. GPU is better on option 1 though. You can add a SSD to option 2 that will blow the 3.4 out of the water for the same price. You will be giving up the 27 panel though for the increased performance.
I personally went with option two. I don't game so the GPU didn't really matter to me. I also went with the SSD + 1TB option as well.
The difference between the 2.8 i7 and the 3.4 i7 is neg. Both turbo up to 3.8Ghz. Very similar performance. You will be giving up a thunderbolt port, and a better GPU, you can daisy chain a thunderbolt port so no big deal.
The reason I went with that setup is that the 21.5 imac is used as my main desktop but not as my main screen, I run dual screen mode and use a 27 Cinema Display as my main screen. The difference between the Cinema Display 27 and the imac 27 is stark in my opinion. You have to see it for yourself. All the 27 imacs I had a yellow hue to them, the colors were way too warm, the Cinema Display is colder, better colors, more accurate. I thing they fixed the issue on the 2011 models so that should not be a problem.
Also you have to consider the fact that using the 21.5 as a extra display opens you up to use whatever screen you want. More future proof. Yes you give up a thunderbolt port and better GPU, but you add the ability to upgrade your main screen whenever you want. New PLS screen, sure, OLED screen with semi-gloss, check. Both of which put the imac 27 and my Cinema Display to shame.
Something to think about.
You can add a 27 to a imac 27, tried that and it just doesn't work well for me, overkill. Way overkill. Can you say headache?