I read somewhere (sorry, can't remember where), that they will support up to to 7GB. But as you can only buy RAM in up to 4GB chunks, not many people are buying 2X4Gb just to throw 1GB away.
So i guess the best thing to do is to buy 1 4GB and 1 2GB, but then you're missing out on 1GB of RAM, which is where the dilemma comes in.
I'm going to go and get a couple of 4GB chips when my local "money black hole" aka computer store - has them in stock.