All 2011 macs have nice performance improvements over previous models. However Aperture is a hardware hog: GPU, CPU, i/o, RAM. Those intending Aperture as an important app will do far better with the strongest available hardware in a given category.
The GPU will be key as to which iMacs are stronger, after that the CPU. RAM should be 8 GB via third party.
The second display will suck (some) power from the less-than-best GPU in the lower end iMacs. Note that every Mac is really a synergy of its parts to run Aperture, so it is hard to say how much effect a second display has onoverall Aperture performance, but I would not worry about it unless using a Macbook Air. I have a 17" MBP plus $300 Viewsonic VP2365wb and the setup works great. IMO for graphics desktop work a second display is a huge benefit.
IMO hard drives are obsolete as boot drives. If I was buying an iMac (I will not, preferring just-as-strong mobile Macbook Pros with their inexpensive SSDs instead) and could not afford the expensive iMac SSD configuration I would get an i5 model and have OWC retrofit an SSD into the Superdrive slot.
Note that iMac hard drives are not upgradable except by Apple so you might as well order a large hard drive now.
BareFeats.com has all kinds of relative graphics performance tests up (more coming), including this link: