I'd definately reccomend that card. I just bought it for my rig (see my sig) and my rig is not comparable to yours, but i can run ArmA on max settings (minus Post processing) 1152*864 on the demo and i get very playable FPS (20+) and im gonna fiddle with a few settings to tweak the last few FPS
It needs a LOT of power though, make sure your PSU is reputable and has 30A or greater SUSTAINED (or 22A sustained on a dual rail PSU) on the 12V rail and i'd reccomend 500W or greater.
But i love it, i payed 150 quid for mine incl. delivery.
This is what's in my tower: Alienware 420 Watt power supply Input: 115 V at 10 Amps 230 V at 5 Amps Outputs: +5 V at 40 Amps +3.3 at 30 Amps (+5 V and +3.3 V combined should not exceed 220 W) +12V at 18 Amps (should not exceed 216 W) -5 V at 0.3 Amps (15 watts) -12 V at 0.8 Amps (6.9 watts) +5 VSB at 2.0 Amps (10 watts) Connectors One 20-pin ATX Main power connector One 4-pin ATX12V Power Connector Nine Molex 4-pin peripheral Power Connector Three Floppy Drive Power Connector One 6-pin Aux Power Connector One 3-pin Fan Speed Monitor Minimum cable length: 20 inches Cable length between connectors: 8 inches Operating Temperature: 50 - 122 degrees Fahrenheit (10 - 50 degrees Celsius) Thank you for your inquiry, The Alienware Technical Support Team