like i said you would need a big gfx card to max out arma 2. the 6870 is slightly weaker than my 5870 but it would give you high settings. not maxed but better than medium... honestly the game is so bugged you could put gtx 680's in and still struggle to get 60 fps maxed out.http://www.bit-tech.net/hardware/2012/05/10/nvidia-geforce-gtx-670-2gb-review/4
hd 5770-gtx 690
this is very high details and your still looking at just 35fps so dropping down to high details would give you a 35fps minimum and about 55 average.
seriously m8 it will punish any card you throw at it. personally i would wait till arma 3 arrives at the end of the year and decide then on a new gfx card if you already have a basic card that can play the game. reason being nvidia will be releasing there gtx770 for around £300-£350 $400-$500, yeah i know thats a lot but you will get the absolute best out of the game and show off your cpu to the best possible standard.
its your choice though... a 6870 now which looks ok and high settings or a 770 later with ultra on the new engine and blow your mind.