I run 2xHD4850 1GB in crossfire and in ARMA 2 it made no difference (in XP or in Windows 7). I would get double the framerate in the cut scenes but in actual gameplay crossfire would knock me down a few frames and cause stutters. And yes, I knew it was running as the obnoxious Crossfire symbol was in the upper right hand corner (I know you can turn it off).
I actually get better frames in the old ARMA 2 missions now by starting from the Arrowhead exe. This is with higher settings as well, and absolutely no stutters. I was thinking about reinstalling everything fresh but I figured if it ain't broke don't fix it (especially with this game). This game is put together with an old engine mixed with new tech. If you are lucky enough to get a good setting "DON'T MESS WITH IT!!!"
Optimizing a game for PC is pretty much impossible as everyone has has different hardware and OS. This is the reason Xbox360 & PS3 games look good even with 7 year old guts. The developers know everyone has the same machine and they have had 7 years to perfect it. This is also why the "new" xbox 360 is nothing more than a shiney new outside with a quiet fan and DVD, the developers would riot if they put in a faster hard drive or DVD drive.
The point is AMD has not released a crossfire profile for this game yet and when they do it will sadly do nothing for performance. You will however get that super cool crossfire badge in the upper right hand corner of the screen (you know, the one blocking the sniper from your view).
Rev