Whether or not you need
8GB of RAM for Windows is an answer I'm searching for myself.
I think there are
situations where you could exhaust your physical RAM, but I don't know when that would be, except during high-intensity video editing, and maybe
enormous still photos. But I'm not sure if any games take advantage of that much RAM.
As for how
you should go about replacing the RAM you have, everything I've read online suggests that it's better to have two banks with 8GB, than running four banks with 2GB. IIRC, it's because of the load all those chips put on the memory controller — the voltage gets increased, which means more heat (at least).
Unfortunately, if you do go with that philosophy, you'll be pulling all the sticks and replacing them with a pair of 4GBs. But if you only replace two 1GBs, I would strive to use similar if not identical speed & timing, for all four modules. Because I'm not sure how well [or even if] dual-channel would run, if you had notably mixed specs in your RAM.