When I was figuring out what to buy (just got my iMac yesterday) I didn't really entertain the 2.8 for what I'd use the computer for, but I did notice that it probably saved you some money to select the 24" 2.4Ghz and then upgrade that to 2.8Ghz, since the top-of-the-line 24" with the 2.8Ghz by default forces 2GB of RAM on you for Apple's overly high price.
Upgrading the 2.4Ghz to 2.8Ghz and then getting 3rd party RAM should save you some money, but not sure what happens to the pricing schemes if you upgrade the HDD, i didn't check all of that out.