Memory upgrade - Late 2009 27" iMac i5

Hi

Am about to upgrade the memory on my iMac and I wondered if someone could advise.

I have the 2.66 GHz Quad-Core i5, with 4GB ram from 2009.
Running Lion. Mostly I use the computer for photo editing, but the occasional bit of video too.
My library is getting pretty big, so am going to install Aperture. I thought perhaps a bit more memory wouldn't hurt. I think that I have 2x 2GB DIMM at the moment, leaving me with 2 spare slots.

Am I able to install 2x 4GB (giving me a total of 12GB) or do all 4 slots need to have the same size DIMM installed?

Also, is 12GB going to be much better than 8GB - Shall I just get another 4GB?

Cheers
Doug

It turns out that the memory is dirt cheap anyway, so I went for the full 8GB.

In case anyone was wondering, the iMac runs super quick with 12GB.
iPhoto has taken on a new lease of life.

Highly recommended upgrade.
Cost only £29 for 8GB. 5 min install.