I've updated the 24-inch iMac Performance (August 2007) article to include Geekbench results for the 24-inch iMac with the Core 2 Extreme processor. It looks like the Core 2 Extreme brings noticeable processor performance gains (take a look at the integer and floating point scores below), but is comparable in terms of memory performance with the 2.4GHz 24-inch iMac (unsurprising, since both iMacs use the same speed memory).
For your convenience, I've done a "mini comparison" between the two current 24-inch iMac models below.
Setup
iMac (24-inch Mid 2007)
- Intel Core 2 Extreme @ 2.8GHz
 - 2.00 GB 667 MHz DDR2 SDRAM
 - Mac OS X 10.4.10 (Build 8R4031)
 
iMac (24-inch Mid 2007)
- Intel Core 2 Duo @ 2.40GHz
 - 2.00 GB 667 MHz DDR2 SDRAM
 - Mac OS X 10.4.10 (Build 8R4031)
 
Results
Overall Performance
| iMac (24-inch Mid 2007) Intel Core 2 Extreme @ 2.8GHz  | 
3791 | |
| iMac (24-inch Mid 2007) Intel Core 2 Duo @ 2.40GHz  | 
3243 | 
Integer Performance
| iMac (24-inch Mid 2007) Intel Core 2 Extreme @ 2.8GHz  | 
3382 | |
| iMac (24-inch Mid 2007) Intel Core 2 Duo @ 2.40GHz  | 
2862 | 
Floating Point Performance
| iMac (24-inch Mid 2007) Intel Core 2 Extreme @ 2.8GHz  | 
5356 | |
| iMac (24-inch Mid 2007) Intel Core 2 Duo @ 2.40GHz  | 
4485 | 
Memory Performance
| iMac (24-inch Mid 2007) Intel Core 2 Extreme @ 2.8GHz  | 
2669 | |
| iMac (24-inch Mid 2007) Intel Core 2 Duo @ 2.40GHz  | 
2402 | 
Stream Performance
| iMac (24-inch Mid 2007) Intel Core 2 Extreme @ 2.8GHz  | 
1998 | |
| iMac (24-inch Mid 2007) Intel Core 2 Duo @ 2.40GHz  | 
1912 |