I have the Sapphire HD5850 2Gb Toxic, which will run the game on the very high settings provided you do not want huge battles, it can easily handle upto around 200 ai with average fps around 25-35, however I recently OC the card using their built-in Auto Tuner which took it from stock 765 & 1125 to 880 & 1280, quite an increase and the same 200ai mission runs at around 10-15 fps faster. I run a Windows7 64bit and have turned the visual settings of Windows to performance rather than windows being visibly attractive just to see if this would do anything, well I was really surprised it did not really add anything to the fps but the game runs really smooth now without those occasional annoying stutters, I can only conclude that it made a performance difference cpu side. I tend to mission smaller skirmishes of around max 80ai or so and the card runs the game great with average 40-50fps, it did very well on stock values, but its that little bit smoother with a little higher fps now its been OC'd..
(quoting the lad that built my Arma2 PC and will be building my Arma3 PC in 2012, if its on time)-'Of course the marry between your cpu and the gpu can make a big difference, you could get a great card but if the cpu does not like it then your in for lower fps etc'..I told him what I was going to want to make missions of ai wise etc, and he kept the price down to a sensible level.
I would be thinking Arma3 as its not too far away, build for that and you won't have much trouble with Arma2..
My settings: (just to give you an idea)
Standard Overall Quality Setting: Very High
Advanced Settings: (left to right)
Tex: Very High
Terr: Very High
Postpro: Disabled (just don’t like the effect)
Vid Mem: Default (or VH)
Objects: Very High
Shadow: Very High
Ratio: 16:9 (my res is 1920x1080)
V Sync: Enabled (monitor is 60hz so possibly holds the average fps back a little)
Edited: 07/11/2011, V sync now disabled, wow the difference in fps... nough said..