Best results can be achieved with windows 7 64bit but don't make the mistake of putting arma 2 on the same physical drive as your windows installation. Making a new partion won't do any good at all, your still relying on one physical disc doing two jobs (arma 2 & windows swapfile / background services) which will make it stutter slot as the hard disc heads jumps back n forth between two jobs. Best way is to hav two hard drives.. One for windows and one for games. This means once ur game is running the drive arma is installed on is doing nothing but streaming arma textures, the other disk takes care of windows requests.
Sorry for long winded reply but people commonly think partitioning can boost performance because it looks like in windows like two different drives. Best possible performance would be using an SSD for your games drive.