Can I install two different capacities of ram on an iMac?

Excuse the earlier post, Ralph Landry1 is correct. Your model does support 4 sticks (up to 32GB) of RAM.

Yes you can have 2x2 and 2x4 in the iMac...be sure and look at your user guide as most iMacs want the modules side-by-side to be the same. So you have two 2 GB modules next to each other and two 4 Gb modules under those but next to each other.

Check with OWC, http://www.macsales.com for quality guaranteed ram...be very, very careful of buying from places like Amazon as often the ram is intended for PCs and not accurate enough for Macs. iMacs are very picky about their ram.

You can only add memory, anything else voids your warranty and all support from Apple.

The Mid-2011 i5 21.5" supports four ram modules, http://www.everymac.com/systems/apple/imac/specs/imac-core-i5-2.5-21-inch-alumin um-mid-2011-thunderbolt-specs.html

As for the SSD, I have heard very good things about this model. http://www.amazon.com/Crucial-2-5-Inch-adapter-Internal-CT480M500SSD1/dp/B00BQ8R HJ2/ref=sr_1_8?s=electronics&ie=UTF8&qid=1390870899&sr=1-8&keywords=imac+hard+dr ive+upgrade Hope that helps!

If you attempt to install an SSD that will void the warranty, only the ram can be changed in that iMac.

Yes, it would void the warranty, but chances are if its a 2011 it has only a limited warranty anyways. I wouldn't recommend doing it yourself though.

You are very welcome.

At the genius bar you will have to buy the ram from the Apple store, they will not install ram that you bring in.

The processor in the iMac is soldered to the logic board and cannot be changed.

The ram at the Apple store is about twice as expensive as from OWC.

If you go to that link I gave above, http://www.macsales.com you can find the prices for various ram options for your iMac and on-line videos on how to install the ram.

On the iMac it is very simple, on the bottom edge of the unit there is a small door held on with two screws. You lay the iMac face down (on a towel is a good idea), remove that small door and then you will see the edges of the two factory installed modules next to each other and slots for two additional modules. You simply have to insert the two modules in those slots, paying attention to the way they go in, there is a small notch that matches a protuberance in the socket. After they are seated and flush with the other two, replace the door and you are ready to operate.

But look at that site and the video to see how this is done.

You are welcome and good luck wth it.