First, having two 2GB cards running in SLI will only give you a combined 2GB of usable memory, not 4GB like it would seem. Second, you only should really have over 2GB if you are GAMING on more than one monitor, usually at least three monitors for Nvidia Surround or AMD Eyefinity. In your case, 2GB should suffice, but usually the 4GB upgrade on a card isn't that much of a pay increase, so it might be just worth it anyways to grab it.
In the end, you do not need the 4GB for your setup. Hope this helps.