I see way too many RAM threads asking the same questions. So if you own a 27" iMac, a Mac mini, or a classic unibody MacBook Pro 13" or 15", here are some places you can buy RAM from to upgrade your system.
Crucial.com: http://www.crucial.com/store/partspecs.aspx?IMODULE=CT2KIT102464BF160B
OWC: http://eshop.macsales.com/item/Other World Computing/1600DDR3S16P/
Data Memory Systems: http://www.datamemorysystems.com/dm50-234-x-2/
As for Patriot, Corsair, etc. I wouldn't recommend them.
Crucial.com: http://www.crucial.com/store/partspecs.aspx?IMODULE=CT2KIT102464BF160B
OWC: http://eshop.macsales.com/item/Other World Computing/1600DDR3S16P/
Data Memory Systems: http://www.datamemorysystems.com/dm50-234-x-2/
As for Patriot, Corsair, etc. I wouldn't recommend them.