Ok just wanted to update this thread as I installed another 4gb last night.
Damn, it really does make a difference to me in battlefield 3, it just feels smoother, there was always a slight stutter that I couldn't quite put my finger on when only using 4gb and having checked ram usage it was using about 3.8gb! So right on the border and probably using a large page file causing the stutter.
The 8gb has definitely smoothed things out, also I now get into the maps with about 25 seconds to go, whereas with the 4gb I would load into the map and already hear all the players shuffling about and running to the choppers just as my loadout screen was loaded up, so 8gb gets a big thumbs up from me!
Ram is so cheap right now that NO ONE should be using 4gb anymore.
Cheers guys.