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...