The memory I bought for my mac mini will not work in an iMac 24-inc.The Ram speed I bought was 1333 MHz . I did a fast check in Google for the spec of the iMac 24 early 2009 and its memory's speed is slower or 1066 MHz. Before you buy ram memory, check Apple's specs for your machine. This is what I did when I bought my memory. If you buy the wrong memory it could damage your machine so be careful.I did see inexpensive memory for your machine,but, again double check before you buy any memory.GK mentioned 1066MHz DDR3 SDRAM and this seems to be correct, again double check.
Best way to make sure exact fit is by going to Crucial's website. Look for your Mac model and it'll give you all compatible upgrades. Then you can go back to amazon and look to see if they have the one you need, otherwise I recommend to get it straight from crucial