I have the same sort of decision to make, although slightly different. My two options are:
20" iMac (specs maxed out) + 20" ACD
24" iMac (specs maxed out)
I can't decide which I like more. Even though I'm not a huge HD guy, that fact that the 20" ACD doesn't have HD is a bit of a bother to me. I do use a ton of Photoshop, Fireworks MX, and Aperture, though, so I'm not sure if two screens with all that screen space on my desk would be to my advantage. What makes it harder is that my two options come out to be pretty much the exact same price around $3,200.
Daniel.