:welcome to armaholic Larsiano.
The gtx285 uses two GPUs if i'm correct, it might be an idea to download the nvidia sli patch or evga fix.
Anyways, a little report from me and my new system 4GB DDR3, AMD Phenom II 965, 4x 3.4Ghz, 260GTX. ArmA II is now a much more enjoyable experience, I am getting more or less the same FPS in multiplayer as single player now. With my new CPU everything is so nice and smooth, however I feel that my graphics card may be bottlenecking the system just a little, but does the job just fine.
I have the same processor and RAM specs as you. I have a 9800GTS graphics card however.
Do you still get "machine gun slow down" when staring down the barrel? It's like slow motion at times (especially the PK machinegun).