No. Your performance result will be somewhat better on 2x4GB as opposed to 2x2GB + 4GB. In the latter setup, using over 4GB of memory will lead to single channel configuration through flex memory - which may need to unnecessary loss of performance since most programs and the Windows OS optimize RAM usage depending on available RAM but ignore whether single or dual channel (not that this optimization usually makes any tangible performance difference anyway; Windows 7 can run smoothly on a 2GB RAM machine, most of the extra is just used for program prefetch - but the drop to single channel will impact performance in some bandwidth intensive apps).
BF3 might just be the first game in existence to actually record a significant performance increase on systems with more than 4GB of RAM, and it is also exceptionally bandwidth intensive; memory bandwidth might just be the reason that Core 2 Quad setups running BF3 just can't perform as well as i3/i5/i7 setups (it was discussed [HERE]) - even i3 setups that are dual core with hyper threading (Hyper Threading provides no difference in BF3, which loves cores) can outperform C2Q based systems on BF3, so evidently bandwidth is very important with BF3 - perhaps moreso than CPU. Perhaps equally. But certainly not less important.
You will see a noticeable loss in performance going with the single channel setup (or maybe you won't notice it, but things might change when you move to a dual channel setup and see a proper FPS boost). Considering your system runs a Phenom II x6 1090T and a 6870 used to run BF3, to go single channel unnecessarily would be foolish - especially considering you don't really waste any more money ensuring a proper dual channel memory setup (the costs are approx. the same at most times).
All Windows apps (well, all apps on pretty much any OS) are preloaded onto memory when available at startup, and many are kept into memory (as long as memory is available) throughout your session to enable faster program and OS start up times, among other things. Ensuring you have a decently speedy memory setup can improve all around OS performance to a certain degree, in addition to games.
Edited by xd_1771 - 1/17/12 at 12:17am