Dude, you are getting blinded by a marketing name “Extreme” there is more to a system than the process (no matter what it is called) so are you looking for the fastest system or the cheapest price?
The newer systems will be faster, by how much, no idea but they have more L2 cache, faster Front side bus and faster memory, all this adds up to “faster” the only down side is the new 2.8 cost $200 at Apple or $100 from Amazon. So which is more important to you a little bit of money or a little bit of speed. There is no right or wrong thing here. If the iMac family of computers meets your needs either one will work so I’m not telling you which one to buy so much as pointing out the differences.
Of course with the new system you can spend even more to get a faster video card or processor.