Note: For the iMac (Late 2009) models, you can use 2 GB or 4 GB RAM
SO-DIMMs of 1066 MHz DDR3 SDRAM in each slot. For the iMac (Mid 2010)
and iMac (Mid 2011) models, use 2 GB or 4 GB RAM SO-DIMMs of 1333 MHz
DDR3 SDRAM in each slot.
As for installation:
i5 and i7 Quad Core iMac computers come with both top memory slots
populated. These computers will not start up if only a single DIMM is
installed in any bottom slot; these computers should operate normally
with a single DIMM installed in any top slot. Core Duo iMac computers
should operate normally with a single DIMM installed in any slot, top
or bottom. ("Top" and "bottom" slots refer to the orientation of the
slots in the pictures below. "Top" refers to the slots closest to the
display; "bottom" refers to the slots closest to the stand).
There is more relevant information in the top link.