Just bought ArmA on the strength of the US demo being playable. My system is 2.3ghz with 1.5Gb Ram and a 7600GS 256mb Card. Not Brilliant, but I've got the eyecandy nice and it's running between 15-35fps depending on action/area etc.
But I have noticed that the controls are a little slow, almost like you're wading though custard. Sluggish I guess is the right word for it. Now, I don't know whether or not this is related to the fact my computer/settings are not good enough/too high or its just the way it is.