Hi, new here, but I've been reading around.
About the extra ram, doesn't ARMA II use 2 gigs tops? (not that it ONLY needs 2 gigs, but the game itself can only utilize 2 gigs as of right now). If you don't intend to do anything fancy wiht the ram, particularly for ARMA II (IE RAMDISK), 4 gigs should be plenty.
What I mean by using RAMDISK is that as of right now, ARMA II tends to stutter even though your are running 40-50 fps at that very instant (usually while youre moving fast). The stuttering is caused by arma trying to load textures from the hard disk and don't seem to get precached. By placing either the whole ARMA II folder or a portion of the frequently accessed files in the game into your RAM via a RAMDISk program, you can reduce or even eliminate stuttering because reading and loading files from RAM is much much faster than reading from a HDD. More on that in this link: http://forums.bistudio.com/showthread.php?t=88388
In short, if you're for sure going to get 8 gigs of ram, I would suggest trying the RAMDISK method mentioned above (you don't have to put the whole game on RAM, just the critical textures mentioned in the thread above) . If you don't want to go through the trouble, 4 gigs is plenty. Note that it doesn't seem to work for all and I have yet have the time to try it, but alot of videos posted show a good amount of smoothing even though frames don't go up. Your call.
ABOUT VIDEO CARD:
here's a quick link to testing different video cards on ARMA II
http://www.pcgameshardware.com/aid,685770/Armed-Assault-2-Graphics-card-benchmarks-and-visual-quality-compared/Practice/
it seems like the game prefers ati cards over nvidia, but suprisingly, the 9800gtx+ you have is doing rather poorly with respect to ARMA II. I think I would upgrade that unless you're doing something like playing FSX, where an older card like the 8800 and 9800 series cards are the best performing of the bunch.
ABOUT CPU:
I think this is a rather CPU intensive game, but between a quad and dual, you can PROBABLY beat out a lower clocked quad with a mcuh higher OC'ed dual core for less, though you definitely won't go wrong with a the one your're eyeing, especially if youre gunna overclock jsut a touch.