What's the max ram I can add to a late 2009 iMac 27"?

hello all..

have late 2009 27" imac.. 2.8ghz, quad core i7.

right have 4gb of ram came preloaded onto computer. 2 gb x 2.

excuse stupid question, not expert on this, if add memory, have stick 2 gb in each of 2 empty slots? or can different. example, 2gb x 2 , 4gb x 2 equal 12gb total. or need go 4gb x 4?

 

16 gb ram maximum, can have 2 two gb sticks , 2 4 gb sticks 12 gb ram.

 

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