Thanks to
this thread on the MacRumor's Forums, I found a perfect mount for my new 27" iMac. It's the
Innovative 7500-1500. Amazon has it for $99 at
this link.
The monitor arm is heavy duty and easy to install. I used a grommet hole on my desk and now the most gorgeous computer I have ever owned floats above my desk like a work of art.
As for the dual portrait monitors, 17" seems to be the sweet spot for display area, but 20" worked better for side-by-side size. However, I've given up on the idea. There's so much room and power I just don't need them anymore.