Clarksville is a Duo Core, Lynndale is a Quad Core.
iMac offers both the i5 Duo Core and the i5 Quad Core.
If you select the lower 27" model with the base price of $1,699.00 you will have the option to upgrade the processor to the i5. The i5 in this model is the Clarksville or Duo Core running at 3.60 Ghz.
If you select the high end 27" model with a base price of $1,999.00 the default processor is a Lynndale or Quad Core i5 running at 2.80 Ghz. You also have the option of upgrading this system with the Lynndale Quad Core i7 at 2.93 Ghz.
For ease of explanation, the difference between the Lynndale i5 and i7 is that the i7 also has hyper-threading which allows 2 processes to run at the same time on 1 core, ie, 8 total processes compared to the i5 Lynndale that can only run 4 processes.
As for gaming, this is very subject. Here is a link to Tom's Hardware which does some benchmarking about cores and they do test out some games and it proves some interesting results.
http://www.tomshardware.com/reviews/multi-core-cpu,2280.html
Bottom line is that if a piece of software is designed to take advantage of multiple cores, it will always perform better on a chip that has more cores. There seems to be a belief in the gaming community that games do not benefit on multiple cores which if you visit the link above you will see is incorrect.
If you dedicate 2-4 hours a day playing games and just want a good graphic experience, an iMac will satisfy you without question. On the other hand, if you were a competitive gamer who actually went for meets and competed for cash, you would want to be running the fastest multiple core processor you could get your hands on and have the machine decked with dual video cards which house the best current chip on the market.
As for deciding on your purchase. It has already been written in this forum that the i7 proves to be the bang for the buck. If that is just out of the budget, consider getting the Quad Core i5. What if next year your favorite game decides to come out fully supporting multiple cores....you'd kick yourself in the butt for not buying the quad as you are now half as fast as you could have been.
For the record, I am not a genius when it comes to this stuff, this is simply information I have gathered and this is how I understand it. If there is anything above that is not quite straight, the good people of this forum will quickly correct me.
troutspinner