For those of you lucky enough to be picking up a new iMac, you may want to bump up the memory a little bit from Apple's standard configurations. Apple is notoriously pricey (though less so recently) when it comes to memory upgrades so you may want to consider a doing it yourself. No worries. Apple has you covered there too. 

Apple's new support document HT3918 covers the easy steps to upgrading your RAM (spoiler - it is exactly how you'd think you'd do it).  Apple requires the same 1066MHz SDRAM that were in previous models.  You can pick this up at Amazon (2x2GB)4GB for $80 or the 8GB (2x4GB) upgrade for about $600-$700.

Pasted installation below:

To install memory into the bottom slot, follow these instructions:

1. Turn off your computer by choosing Shut Down from the Apple () menu.
2. Disconnect all cables and the power cord from your computer.
3. Place a soft, clean towel or cloth on the desk or other flat surface to prevent scratching the LCD display.
4. Hold the sides of the computer and slowly lay the computer face-down.
5. Using a Philips screwdriver, remove the RAM access door in the bottom of your computer.

   

6. Remove the access door and set it aside.
7. Untuck the tab in the memory compartment. If you are replacing a memory module, gently pull the tab to eject any installed memory module.

   

8. Insert your new or replacement SO-DIMM into the empty slot, keeping in mind the orientation of the keyway of the SO-DIMM as shown below.
9. Once inserted, press the DIMM up into the slot. There should be a slight click when the memory is seated correctly.

   

10. Tuck the tabs above the memory DIMMs, and re-install the memory access door.
    
11. Stand the computer in its upright position, reconnect the power cord and all other cables to the computer, then start up the computer.

Update - Crucial has the 8GB set for $640