robotkiller
macrumors 6502
I don't know your use case, but for me - I'm being constrained because of a lack of memory, and not CPU speed.
I have a similar iMac (Late 2006 20 inch screen, 2.16 C2D, 2GB RAM) running Leopard and a Unibody MBP (17 inch, 2.66, 4GB RAM), and I'm dealing with the same lack of RAM with photo editing on both.
The iMac will cost 350$ to get you to the 8 GB level, and God only knows how much for the 16 GB zone. What is making me think of a hackintosh or MacPro is the ability to add RAM in the amounts that I need.
The 27" can be bumped to 8GB with a $117 kit from OWC.