Dual displays on an iMac IMHO are tricky since it's nice to get a matching display set, and if you try to accomplish that with the iMac you're seriously limiting yourself. I guess the good news is you haven't bought the iMac yet, so you can think about other purchasing options.
IMHO the iMac monitors are good but far from the best available for photography. First up is the glossy screen. I'm not going to get into it here (been done to death) but make sure glossy works for you, and your environment.
The next big killer is the "limited" gamut of the iMac screen. Wide gamut screens have real benefits for editing photos, and it would be money well spent to get a model with wide gamut.
After that it's mostly smaller things, but which further differentiate: 10+ bit capability (if OSX ever supports it), internal LUTs, etc.
Maybe instead think about getting the cheaper 21" iMac, and then getting a nicer 27" separate LCD to use as your main? That way you can concentrate the quality in the main monitor and not the secondary.