I'd look at a new 27" iMac and installing a 6G SSD in place of the internal optical drive (if the latest iMacs still need you to remove the optical drive to install an SSD) and use the SSD as your boot applications drive . It comes with 4GB RAm standard but if you've got the money you can go up to 32GB RAM.
I run the following photographic processing programs with my 2.66Ghz i5 quadcore iMac with 16GB RAM (OWC) and an OWC 3G 240GB Mercury Electra SSD as boot/applications drive:
Lightroom 4.1, Creative Suite CS6 (including PsCS6 obviously) CaptureOne, PTGui Pro; Helicon Focus Pro, ColorThink 2.3, and Datacolor and Xrite color management programs
My cameras these days run in the 17 Mp range and up to 39mp.
The system is very fast and I can only imagine that with the current 27" iMac processors and the 6G SSD it will be even faster.
I had the SSD installed by a local Apple authorized repair center here in Atlanta but OWC can do the job for you if you have your iMac shipped to them. The details are at http://eshop.macsales.com/shop/turnkey/iMac
If you haven't discovered it already http://www.macperformanceguide.com/
is a very reliable source of information.