The iMac has two RAM slots, but if it comes with 2GB, it will have two 1GB sticks in it. When I bought my iMac in late 2007 and upgraded to 2GB, they took my two 512MB sticks in payment (I bought the iMac and let the store upgrade to 2GB right there).
Or did Apple have a different RAM policy back then? Meaning it comes with one 2GB stick if they sell it with 2GB RAM? Every Mac I bought with two RAM slots, had its RAM capacity filled by two sticks of the same size, never by one stick with the maximum size. Even the iMacs with four slots and coming with 4GB have two 2GB sticks in them.