I have just thrown an extra 512Mb into my 20" iMAC to take it upto 1Gb. I dont really do anything serious on it, just safari, mail, itunes (admittedly with a quite large database), flash8, iPhoto and a highly recommended app called "clutter" . But you can really tell the differnce it's got loads more zip !! If anyone is wondering if it will help, just buy it !! I got mine from Crucial only about £35 quid.