Before anyone jumps off a bridge...
... here is a quick and dirty comparison of the old and new graphic cards in the iMacs. Disclaimer: the closest I could find to the GeForce GT120 was its predecessor, the 9500GT, which was apparently recently rebranded as the GT120.
Old GPU:3DMark,New GPU:3DMark,% Increase
Radeon 240: 144, GeForce 9400M: 439, 204.86%
Radeon 2600Pro: 630, GeForce GT120: 1222, 93.97%
GeForce 8800GS: 3614, Radeon 4850: 6830, 88.99%
These are independent scores from
futuremark.com.
Keep in mind that in OS X 10.6, Apple will be offloading many tasks onto the graphics processor. As a result, the main focus for the new iMacs has been on upgrading their previously pathetic grahics processors to ones that could be considered more mainstream. So, my initial disappointment has turned to interest in the second from top model, with the graphics upgraded to the 4850, which would yield about a 600% increase from the graphics performance of my current machine. Also, base RAM, maximum RAM and HD size have all doubled.
So, yes, the new iMacs are hardly revolutionary, but they have some significant improvements from the previous batch, and they are likely created with OS X 10.6 in mind.
Edit: sorry for the terrible formatting, but I can't seem to get the columns right.