I went from a Dual 2G to a single 2G iMac - with more RAM on the single 2G. Frankly, I find the single as fast as my Dual 2G was in MOST cases, with exceptions when I'm really using the computer hard (encoding DVD + Photoshop tasks + all the other stuff running). My main reason to move to a single iMac instead of my PMac was for space, quietness, and heat...in my office it got a bit more important.