what CTF and i were discussing was the trade off between having 4 x ram slots on the A55 model chipset for the socket FM1 or having high speed interfaces (SATA 6Gb is for solid state drives or hard drives and USB3 is for external devices such as webcams or external hard drives) on the A75 chipset.
Basically the problem with buying a single 4GB stick of ram is that you dont benefit from the full capability of the fusion platform as having 2 x sticks of ram allows both sticks to be accessed simultaneously (dual channel) thus giving you greater speeds (bandwidths). If you got 2 x 2GB sticks you would have the full speed benefit however if you were using a motherboard that only had 2 ram slots, you would have to replace the existing sticks rather than adding more when you decide to upgrade.
Another thing that CTF pointed out was one of the major companies involved in supplying chips for RAM resellers might be going bust, this would push ram prices up which might make it more expensive to upgrade at a later date. For that reason, and considering 8GB is only £35 at the moment, it seems wise to just get the whole lot now so that you wont need to worry about your RAM for the foreseeable future.
Whether or not you decide you would like to have the extra capability for more ram than that is up to you, although it seems pretty pointless to me, i have 8GB at the moment and windows doesnt use anywhere near all of it.
Does that make sense?