I'd call and verify, as what's listed is really PC memory (standard = no thermal sensor), but with the 4GB sticks, Apple specified them (non-standard).this is the ram i have ordered and i cant see that it says anywhere on the website that i cant mix it with my 1gb dimms.
http://www.crucial.com/uk/store/mpartspecs.aspx?mtbpoid=956B1B54A5CA7304
Worth a call anyway (I didn't find a specification .pdf linked on that page to find out more information).