I think the extra $400 for the high-end iMac is a waste. What I would do is buy the middle range 24" iMac. Upgrade the graphics for $200 to the ATI 4850, that would rock. Then, in a year or two when RAM prices fall out of the sky, load 8 GB of RAM into it for less than $100. That will make it a speedy computer for several years.
Unless you are doing lots of "pro" level apps, the power of the Mac Pro will not benefit you AS MUCH. However, a Mac Pro is still the way to go in my opinion... if MONEY is NO OBJECT.
Obviously, the low-end Mac Pro would kick some serious ass over the iMac. The Nehalem Xeon CPU rocks... if money is really no object, might as well buy the high end. If money is really REALLY no object, load it with RAM. If money is really REALLY REALLY no object, put dual Quad 2.93 GHz chips in it and deck it out all the way, RAID and 15,000 rpm drives and all.
However, if it comes down to what would be fast or a good price value, I think the iMac does a pretty good job with a nice IPS display, and the future potential to upgrade to 8 GB RAM for much less than the current price. Add an SSD drive for some more speed. And I think that computer would be a lot of fun. But if you want truly sick performance, buy the Mac Pro.
Going back to the first post though, the new Mac mini with a 30" ACD would do the job... for most. Especially with maxed out RAM. The nice thing about mini is it can always be used as a media server later. The nice thing about a 30" ACD is it can always go with a Mac Pro later. I would definitely stick with ACD. Also, I own a 24" ACD, and I use it with an MBA. It works awesome. It's a lot less money and for $899 you get speakers, a USB hub, and video camera included. It would work great with Mac notebooks too (new ones anyways).
Lastly, Snow Leopard is coming out soon, and it could further make a case for the POWER of the new Nehalem Mac Pros.
Good luck whichever route you go.