iMac Memory Upgrade

Well, you'll need to know the memory specs first. Look up your machine on http://everymac.com. I see two that fit your description, a 3.06 GHz and a 3.33 GHz model. The former specifies RAM as follows:

RAM Type: PC3-8500 DDR3
Min. RAM Speed: 1066 MHz
Details: Supports 1066 MHz PC3-8500 DDR3 SO-DIMMs (204-pin).

Then I'd also go to one of the big manufacturer's websites (like crucial.com or kingston.com) and use their memory selection tool to make sure its recommendations match up with everymac.com as far as the specs.

For brands, I've had good luck with OWC, OCZ, and Patriot and bad luck with PNY, for what that's worth.

You might get more responses if you add "late 2009 27 inch iMac" to the thread title...

 

I have the same model iMac and added Crucial memory when I got the iMac. I've had no problems with the memory.

 

Each slot can take either 2, 4 or 8GB - so with all slots filled now, you would remove whatever you are upgrading.
I like OWC or Crucial, and often use Kingston. Any that have a lifetime warranty is a good choice.
An advantage for OWC is the rebate - they will buy your old RAM, so would drop the cost.