I have an iMac (and now I have exhausted myknowledge of my machine). It came with 2mgs of ram and I have added 2 because I found PSCS3 not quite quick enough. I think it might be a matter of personal preference. I have no patience. I can tell you that I bought non-Apple Ram at a fraction of the cost Apple sells it for and that I was able to install it myself in under 5 minutes even though I can't tell you anything else. I run iTunes, PSCS3, Multimedia Express and a few other programs simultaneously every time I sit down and I have not found it to be a problem.
1GB can be very laggy at times for me. I STILL need to upgrade. If i can get 4GB, I will finally be able to work on GarageBand and PH at the same time!! I also noticed that ScreenFlow is another memory hog.