I wanted to upgrade my iMac RAM from 2GB to 4GB. The very first thing you need to do is identify which Model and year iMac you have (instructions for upgrading RAM vary for different model iMacs). I found that mine was – iMac (Mid 2007). Instructions to find which iMac you have can be found here on the Mac Website Then you need to order the RAM. I found this very confusing at first but then I found Crucial (there is a UK and US version of the website).  
You can download a file which scans your Mac and tells you exactly the RAM you need. I also double checked in my Mac System Profiler [Apple Menu – About this Mac – More Info]. If you look at Memory you can see as in the screen shot above (this picture was taken after the upgrade). The speed of the RAM is very important. So if you check the existing RAM you should find the speed. Unfortunately I didn’t take a screen shot of System Profiler before I upgraded but I thought it also said PC2-5300 somewhere in the RAM Description (it doesn’t now).
From Crucial’s Website:
Please check the memory frequency of your iMac. Your memory upgrade needs to match the frequency of the installed memory. Note that the memory speed and the bus speed can be different.
The speed of the installed RAM matched the scan results from Crucial so I went ahead and bought this from Crucial (details below).

Module Size: 4GB kit (2GBx2)
Package: 200-pin SODIMM
Feature: DDR2 PC2-5300
Specs: DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 256Meg x 64 •

Mac Face Down

Instructions from Apple (found here)

1.Turn off your computer by choosing the Apple () menu, then Shut Down.

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.
Using a Philips screwdriver, remove the RAM access door in the bottom of your computer.

Remove RAM Access Door
Unscrew RAM Door with a Phillips Screwdriver
RAM Access Door Opened

Tabs that hold in RAM

Tabs that hold in RAM
Apple Instructions – Untuck the tab in the memory compartment. If you are replacing a memory module, gently pull the tab to eject any installed memory module.
My Experience: Took a while to pull out the memory I didn’t wan’t to pull the tabs to hard. The tabs can be slippy so avoid sweaty hands. I would say pull firmly and it eases out.

Orientation RAM
Orientation of RAM

Mac Website shows a diagram for the orientation of the RAM. The keyway should be as in this picture – basically with the nick/gap to the top left before you insert it. Both slots are the same i.e. the RAM is inserted in the same direction for each slot.

[In the previous picture to the one above (Tabs that hold in the RAM) – to the left of each slot you can just about see the corresponding piece where the RAM slots in.]

RAM Inserted
RAM Inserted

Mac Website says you hear a click. I did not hear a click. I think I was being a bit of a ‘girl’ and too gentle – terrified I would damage it 🙂  I eased it in and then applied pressure gently. Maybe if I pushed more firmly it would have clicked. I knew/hoped it was in right when it was aligned as in the picture above (and also 2nd Picture which shows what the RAM looked like before I pulled it out) and I really could not push it in any further.

Re-Instert Tabs
Re-insert Tabs

Just re-insert the tabs and screw the door back on. After I started up the Mac I double checked in System Profiler and all was OK. It was showing 4GB of RAM (very first picture). Hoorah!

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