Just about to order a NEW iMac, and am really pleased to see that the cost of adding memory has come down. Can someone tell me if there's a performance benefit in getting my 8GB as 2 x 4GB rather than 4 x 2GB - one is definitely a lot cheaper! I assume the benefit of 2 x 4GB is that there are still two free slots to go up to 16GB at a later date?