Memory Increments for iMac

What I'd like to know is how can I fill the two empty slots of RAM available on my iMac.

Do I have to stick to 2 GB additional chips or can I place two x 4Gb on each so I make it 12 Gb RAM? Are any combinations possible, like 1x2GB + 1X4Gb ?

BTW: what type of memory do I have to look for?

This is my iMac:
Model Name: iMac
Model Identifier: iMac9,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2.66 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 6 MB
Memory: 4 GB
Bus Speed: 1.07 GHz
Boot ROM Version: IM91.008D.B08
SMC Version (system): 1.45f0

Max ram for your iMac is 8 gig.

To keep things simple...just install 2 x 4gig sticks of ram = 8gig.

Good to go...ram is maxed out.

- Nick