Upgrading RAM on 2011 iMac.

Hi, I've had my iMac for about 4 months now and have decided to upgrade my RAM as it is relatively cheap to do.
I've been watching videos on Youtube and it looks like an absolute breeze to do. My question is, I'm sure my iMac has 4Gb of RAM, and from the videos i've watched it looks like it comes in the form of 2 x 2gb sticks. But it appears to have 4 slots, can I just buy 2 more 2gb stick and slot them in along side my existing RAM? If so, could I perhaps buy 2 x 8 gb sticks to up my RAM to 12gb? OR do you have to used exactly the same RAM sticks for each of the slots? A little confused!!

Your iMac has two memory bays: One upper and one lower. The current 4 GB of memory is installed in the upper bay as 2 x 2 GB modules. You can add 4 GB to the lower bay with 2 more 2 GB modules.

However, if you buy 8 GB (2 x 4 GB modules), you'll need to remove the 4 GB (2 x 2 GB) from the upper bay and move them to the lower bay as the larger modules should be placed in the upper bay. I hope you're following....

One other thing you need to know, the modules are seated very tight in an iMac and can be difficult to remove. Also inserting modules is likewise difficult as the slots are tight. It takes considerable force to seat them properly so be careful.

I installed 2x4gb ram into the already empty slots (giving me 12gb) and did not move the other two, and it works fine. They were hard to push in though, needed some force.

That's fine, they'll work OK in the lower bay. However, the recommendation is to put the larger modules in the upper bay.

I purchased an iMac yesterday and was also looking into upgrading the RAM, considering how cheap it is

I'm seeing conflicting information on what the max is though. According to Crucial, 8gb is the max.

According to the Apple support page, it's 16gb for the Mid 2011 models iMac (Mid 2011): Memory specifications

But according to the Specifications page of the iMac, it depends on the model Apple - iMac - Compare the 21.5- and 27-inch iMac.

How can I figure out what's the real number?

That's correct. Apple states the Mid 2011 iMac models (21.5 and 27) have a maximum memory capacity of 16 GB. However, Mactracker which has always been correct states that the max for both models is 32 GB. Likewise Other World Computing says the same thing. (32 GB)

So who's lying about the RAM? Apple or Mactracker? I owuld love to have 32 in my iMac.

Apple is always going to give the more conservative figure for maximum memory. Both Mactracker and OWC (see link above) have tested the machines with 32 GB installed. You have to realize though that putting 32 GB in an iMac means 4 modules of 8 GB each. Very expensive. $$$

Indeed it looks quite expensive for 32gb

16gb however is relatively cheap, so i'll be doing that

Hmmmm - looks like the 8G modules have come down a bit. Might be tempted to put 16G in the MBP since I seem to be very proficient at eating up the 8G that's in there now.