Upgrading my ram, couple questions

[Nexile]

I've slowly been keeping my PC up to date. RAM, CPU and my mobo are starting to lag behind now though. Specs are:

ASUS P7P55D-E (I think)
i5 750 @ 4ghz
4GB G.Skill DDR3-2000 (PC3 16000) G.SKILL-Products
sapphire HD6870 (Adding another for crossfire very soon)
corsair tx750 PSU

I wanted to upgrade my ram for BF3 (If I even need to, I could hold off on it, I just feel my ram would be lacking after I get my second gpu.) I had planned to just get another 4gb of the same ram but I built my pc a couple years ago and can't find it on sale anywhere now. So my questions are; should I just try to find a similar 4gb kit and will they work together?
For example:
http://ncix.com/products/?sku=42745&...l&promoid=1055


I've heard the mobo will just use the lower frequency ram but would the speed be a problem? And how would this effect my OC? trying to find similar 2000(16000) ram is ridiculously expensive.
Would I just be better off buying a whole new 8gb kit or is getting another 4 the right thing to do? Is there any performance differences to using 4x2gb or 2x4gb?

Basically all I would like is someone to recommend me which ram to buy (preferably sold at ncix) and tell me how it might effect my overclock. I'd like to think I know a decent amount about computers but I feel lost on ram (mobo's as well ;p )

Thanks for any help, sorry for the wall of text.

[LarkStarr]

as long as it's similar, it should be perfectly fine. it might take a slight more effort to overclock properly if you do overclock the ram, but otherwise all your other questions would be answered with a 'no / don't worry about it'

and from what I can tell, BF3 isn't a ram hog at all anyways. you might be just fine @ 4GB

[supaflyx3]

BF3 is indeed a RAM hog, i have troubles playing it with 4GB of RAM. I get stutters because of it quite often, as my ram usage is at 100%. This is with a fresh install of windows, with nothing but chrome & irc running.

[LarkStarr]

I'll have to check myself again later, I don't remember when I checked, maybe the game wasn't fully loaded.