Looking into buying an iMac. I already have a 20" Apple Cinema Display (ACD) and was thinking of buying a 21.5" and just using the ACD as a second monitor attached to it. Of course, I could also buy a 27" and sell the ACD. Anyone have any experience with one or both of these configurations and can recommend one or the other? Even if you don't have experience with either config, your opinion is still appreciated. Thanks.