Performance difference between 2.4Ghz v 2.8Ghz

Honestly, .4GHz difference won't be very noticeable. You would notice a difference between a 2.0GHz and 2.8GHz, but .4 isn't that much. When I purchased my iMac the choices were a 2.16GHz and 2.33GHz, a very slight gap. In my opinion it wasn't worth the extra money.

If the two CPUs had different specs on bus speeds, I would consider the upgrade. Otherwise, I see it as a merely "nice to have" upgrade. The VRAM and RAM is where you really want to put your money because they'll make the largest impact on running the OS and applications quickly.

Not allot of difference but a big difference in price.
According to MacWorld they have tested the speeds of these two processors and found that there was a difference of 10%.
10% is a very minimal difference.

MacWorld quoted>>So we put the 2.8GHz iMac 
through our battery of tests and found it tallied a 
10-percent improvement over the Speedmark score of the 2.4GHz model.

The benchmark score overall was for the 2.4 Ghz 277. The 2.8 Ghz score was 304.

Depends entirely on the app. For compute-intensive work, with little or no access to main memory or disk, you'll see +no more than+ 17% difference (i.e., 2.8/2.4 = 1.167).

For main-memory intensive apps, you won't see half of that -- maybe 5% or so.

For I/O intensive apps (e.g. a web server), there'll be ZERO difference.

...as always, the answer is: "it depends."

Looby

Here are MacWorld's "Speedmark 4.5" numbers for recent iMacs.

ALU 2.80 GHz .... 304
ALU 2.40 GHz .... 277
C2D 2.33 GHz .... 266
ALU 2.00 GHz .... 253
C2D 2.16 GHz .... 245
C2D 2.00 GHz .... 232
C2D 1.83 GHz .... 202

...FWIW, ...ymmv, ...batteries not included,

Looby

Aside from a larger hard drive as stock the two machines are virtually identical except for the different CPUs.