OtherWorldComputing (OWC) published an interesting test illustrating the impact of the amount of RAM installed in the new iMac on performance. All models have been tested with 1 to 4GB of RAM.
In summary, the default configuration equipped with 1GB of RAM will work fine for basic and single task usage. If you intend to routinely have several applications opened simultaneously, you will install 2GB of RAM.
Adding 4GB of RAM would make sense for users intensively using RAM-eager applications such as Photoshop CS3, however, such users would be most likely be working on a Mac Pro.
Considering the price of RAM modules today, it might be a good investment to install 2GB or even 4GB of RAM by default in order to have the new iMac fed with a large amount of memory for its lifetime; nobody knows how Leopard will "consume" memory.
[translation by
