I added 1 GB of RAM to my iMac, bringing the total up to 1.5 GB. It is much more responsive now. Applications launch faster; the Dock pops out instantly when I put the mouse in the right place; windows resize without all the jerking around; Exposé is smoother.
If I had it to do over again, I would have ordered the iMac with the minimum RAM (256 MB), then bought 2 GB of RAM from someplace else. Crucial will sell 2 GB of Mac RAM for $505. In contrast, Apple adds $1,225 to the price tag if you want 2 GB out of the box.
Performing the upgrade was easy. I did some bad things to the screws when opening up the Mac. I was expecting them to come out when I unscrewed, but they are captive screws that stay in place when you turn them. So I ended up unscrewing too far, and turned the Philips slot into a conical pit. Don't make this mistake if you do this yourself.