What RAM modules will it hold?

http://support.apple.com/kb/HT5523

This lists the specifications that the RAM must comply with to be compatible.

http://support.apple.com/kb/HT5540?viewl...

This shows the procedure to upgrade the RAM.

The 27" and the 21.5" iMacs are different in that the 27" has a dedicated RAM compartment, accessible from the rear WITHOUT tearing open the screen.

Hope this helps!

EDIT: To find out what RAM your computer will work with go to http://www.crucial.com/uk/systemscanner/

and download the tiny application (Windows or Mac).

After it's run it will send you to a webpage with an analysis of your computer and links to buy compatible RAM.

Nobody posted just how easy this upgrade is:

27英寸 iMac（EMC2546）内存更换

" I see that many high-performace modules have heat sinks or heat dissipators on them. Is it likely these will fit into the new slimmer "

They will not fit nor are they necessary as they are adequately cooled by design.

For installing or replacing memory iMac (27-inch, Late 2012):

http://support.apple.com/kb/HT5540?viewl...

From the question "I see that many high-performance modules have heat sinks or heat dissipators on them."

Keep in mind that iMac's use SODIMM memory (often referred to as Laptop) for at least the last 10 years. The high performance modules you are referring to are desktop PC memory.

Also starting with 2012 models the 21" iMac is does not have a memory access plate. To not void your warranty you would need to take the 21" unit to an authorized service center or to apple to upgrade a 21" memory. The 27" iMac still has the access plate and is therefore user upgradable.

If you ordered 21.5" new iMac, unfortunately you would hardly be able to upgrade RAM that it will take 1600MHz DDR3 memory. I don't mean you can't but it's a big task. The new iMacs don't have Memory Room on the bottom of the housing that you could have an easy access with a screwdriver. Now that Apple has gone farther towards thinner framework, the memory resides inside the housing making it difficult to play with. You have to use heat-gun to pry-up the front glass just to access the memory. The LCD and glass are infused together. Boy, you have a lot of work ahead. If you broke the glass while prying, you have no idea what costs you are inviting in. It's frustrating. Here's the teardown that may help you locate and decide if you wanna do it.