The RAM used in the iMac is all standard stuff (although I'm not sure if it's SODIMM or regular desktop RAM? Can't be bothered to look it up!) which can be bought from a number of manufacturers, often cheaper than from Apple themselves. RAM installation on the iMac
could not be simpler, and there should be instructions in the user manual on how to do it, too.
You sure can run dual-screen using the output — extending your desktop across at different resolutions, not just stretching or mirroring. Although bare in mind the fact that it does not have a DVI output — it has a
mini-DVI output. You will need to buy a DVI or VGA adapter (sold separately by Apple) to hook up an external monitor, and the DVI adapters supplied by Apple cannot be used in conjunction with DVI->VGA adapters either.
*av