There are many factors that affect Diablo performance and not all of it is related to the computer. Issues include network congestion and server load.
As for local stuff. How much RAM does the person have? what else are they running at the same time. Are they running it at the highest resolution with the maxed out graphics settings?
Also, finally, everyones perception of "runs fine" is not the same. I think a 2012 iMac (if/when it's released) should be just fine for Diablo 3. As for getting a maxed out system or not... how long do you want your system to be good for your use? I think the high end base model with the 2GB VRAM and adding at least 8GB of third party memory. (Why 8? Based on the prices of memory, it's a good price point.)
I'm not sure I've seen any reports on how an SSD affects Diablo 3.
I am the person in question here, and I have standard base model 21.5" (5 days since purchase) and I'm yet to see any issues on Diablo III asides from when someone else in the house is streaming or using dropbox on the network, I've had stuttering twice whilst playing and both times asked my housemates if they were doing anything on the network and those were the reasons.
My specifications are base, 4GB RAM and standard HDD setup, I do however close everything on my system whilst I am playing Diablo III so that it is the only thing running.
Using the Max FPS limiter to 60 FPS Max Foreground Frames Per Second and 60 FPS Max Background Frames Per Second I usually see a fluxuation whilst playing of 30 - 40 FPS in 'intense' moments with 3 other players in Public games, and a steady 60 FPS at all other times, I am yet to see it drop below 30 FPS at any moment which has been absolutely fine and playing smooth for me! And yes, I am running it at the max settings with VSync turned on, Anti-Aliasing etc all on too!
As for other things I have installed on my system (incase anything on there reserves memory or CPU/GPU usage regarding 'helper' programs etc) I've only installed Diablo III, Adobe Master Collection, Adobe Muse, Coda 2, CarbonCopyCloner, Skype, Adium, Spotify, XBMC, Twitter, VLC, Things and Microsoft Office 2011.
I rarely install anything other than that on any of my systems!
I hope that helps, if the OP is buying with the intent of gaming regularly however (I am not an avid online gamer, my brother just bought me a copy of Diablo III to play alongside him. The only other things I play are Portal and Humble Indie Bundle releases) I would still advise them to go for the higher specification 27" they initially planned on, as whilst I find my base 21.5" works fine for this game, I know it may struggle with any upcoming games in the future which I presume may be of more interest to the OP than myself!
