I'm still busy moving into a new house, but did spot this review looking at the new ATI 5850, which should sell for around $259. They tested it with ARMA 2 and came away very impressed. It did better than the more expensive Nvidia 285 and the ATI 4890. They also use the tests done with the 5870 last week to compare it.
AMD's ATI Radeon HD 5850 Video Card Review
In this case, the Radeon HD 5850 single video card was playable at 1680x1050 with visibility at 1600, Antialiasing on "Normal" and everything else on "Very High." This provided a very rich and detailed gameplay experience in Arma II. This is close to as good as you can get in this game, other than maxing out the visibility slider, which is impossible on any video card right now.