From Arma 3 Forums at Bohemia Interactive Studios: Link
Arma 3 stratis 1600-2133 15.369 % increase 
Arma 3 altis 1600-2133 11.477 % increase
More RAM is not as important as Faster RAM with better timings. Arma 3 will easily use 6GB of RAM so 8-16GB should be fine so long as you are not opening a bunch of programs.
Lol :P. I get ~20fps @ 2560x1440 in multiplayer, sitting in spawn, staring at paint on a wall. This is with i5 750 @ 3.5Ghz and ram @ 1600Mhz, I have 2 R9 290s @ like 1050Mhz. This is with latest drivers. Its clearly completely broken or I have some sort of weird hardware compatability issue.
I don't think it matters how low I turn all the graphics settings either. Its pretty absurd, its quiet a bit better (like no problem at all) in singleplayer, but they probably just have an appauling game engine.
Imho the VR Engine is far from beeing "completly broken", it may have its flaws and coulbe be more optimized for Multicore and such. For me the biggest problem is also the biggest feature: Freedom. The freedom to create anything you want (sort of). And with bad design of a mission or addon you can break your games performance so easy. And I dont think it is the games fault someone is running a 100 player COOP mission with 2000 AI and only gets 5 FPS... Just because the game doesn't prevent you from going all out crazy should not mean the engine is broken.
I personaly play on a FX-8320 @4,3 GHz and 8GB @ 1866 MHz and a HD 7950 (Windforce stock) and a SSD.
And in the MP missions I play / create / lead I mostly sit around 30fps and I am happy with this. This is with mostly high settings and a object viewdistance of 1500. We are mostly around 15 players, have a bunch of mods running and play COOP with a reasonable amount of AI.
I got a 4790k @ 4.6GHz, 16GB 2400MHz RAM and dual 280Xs, playing at 1440p, I get decent frame rates in Campaign and scenarios but multiplayer takes a complete crap on my system, and im the type that doesnt like to sacrifice quality lol so I just dont play MP in ARMA.
the problem
The problem you have is that the game is NOT GPU heavy. It doesnt matter if you have 2x 290s. You CPU is ancient as all hell. Seriously, an i5 750? That is a first Gen Core i5. It is a Lynnfield. And your RAM is only at 1600mhz. What does the OP state? Faster RAM and Faster CPU = FASTER FPS. Also, turn off AA unless you set it to the lowest setting. I play Breaking Point on Arma 3 with ~75 players. Very little lag and excellent frames. No noticeable frame drops except when new stuff spawns in every once in a while. I can not OC my setup at this time because every time I OC my CPU, my AIO Cooler fails and my PC BSODs. My H100i has been dead for a while but as long as I dont stress my CPU, it "works". Im saving for an NH-D15 or similar.
Here are screen shots I took ingame:
Have you tried recently? And also, what was the ping to the server? What mission did you try to play? These things matter. something like King of the Hill or Invade and Annex should not be an issue. They are very well optimized and written missions. One thing you can do is reduce the view distance to about 1500-2k.