Q: iMac 24" Early 2009. 4GB RAM to 16GB upgrade?

Ok, I do have one question to verify your specs in order to view the upgradable amount of Ram. Is your iMac intel core i3 or Core 2 Duo as Apple didn't release i3 on their iMac model until the Mid-2010 models.

An early 2009 24" iMac is only capable of using 8Gb of ram.

IF you have a mid 2010 Core i3  iMac it can use up to 16Gb of ram. Only the i5 and i7 iMacs can use more than 16Gb.

When upgrading ram I suggest using a quality vendor such as OWC https://eshop.macsales.com/shop/memory/iMac/2010/DDR3_21.5_27

Or Crucial http://www.crucial.com/ and avoiding other bargain basement brands.

The processor can't recognize more than 16 Gb of Ram, therefore you would be wasting your money by installing 4x8GB sticks of Ram.  However, you could install 4X4GB of ram sticks to truly maximize the performance of the Mac. Personally, I have the same iMac and I recently installed 2 more 2 GB sticks for a total of 8 GB of Ram...and for internet, office, photoshop and emails, it works exactly as I need it to and I believe if I were to have 16 GB installed, there wouldn't be much of a difference.

I don't use anything other than Crucial or OWC ram. Both are competitively priced, guaranteed to work on your Mac and offer excellent customer service.

Over the years I have bought a lot of RAM for quite lot of Macs (hundreds, approaching a thousand). Crucial  and Apple are the only brands that I have found work every time.

I've never used OWC as a supplier but people I trust have and they tell me they are as good as Crucial.

Buying cheap memory is like buying cheap brakes and tires on a car. Everything seems okay for a while until...

If you want to save money, upgrade to 8GB first and see how you go. 8GB is enough for most people unless you are running virtual instances of Windows, etc.

C.

Macs require exact spec ram. Those "other brands" play fast and loose with the specs and consider close to be good enough. PC's can use ram that is close but not exact, Macs cannot. If you want to buy another brand go ahead, just remember that the sweetness of low price can soon be replaced by the bitterness of kernel panics and possible data loss or corruption.

Looks like the Late 2009 iMax with i7 processor (iMac 11.1) will support 32 Gb ram.

https://en.m.wikipedia.org/wiki/IMac_(Intel-based)#Aluminum_iMac

Earlier versions (iMac 10.1) support 16 Gb...

My favorite place for memory is Data Memory Systems...

http://www.datamemorysystems.com/apple-imac-intel-memory-upgrades/

More info...

http://www.everymac.com/systems/apple/imac/imac-aluminum-faq/imac-intel-aluminum -21.5-inch-27-inch-how-to-upgrade-memory…

I upgraded Ram in many Macs over the years. Now although I don't stick with just OWC and other tops brands because there happened to be less expensive ram from other brands, but I wasn't dumb with my decision either.  Obviously if I had never heard of the brand, or if they have low ratings of unreliability, then I'm not going to purchase it. However, there are other respectable brands that many users have success and reliability with.

http://www.tomshardware.com/forum/45556-30-ranking-best-brands

(This link here provides information on the best ram brands after many experiences. My most useful information from this post came from the third reply by 10GHZ and a reply made on November 5th by pcdinesh)