@ Josh,
The controls and movement are vastly superior to Arma2, that was my main annoyance with Arma2 but this alpha is excellent and I would be inclined to say this is actually better than BF3. You need to look at them as totally different games really, BF3 is great but its less realism more arcade, Arma is all about realism and it certainly has hit the nail on the head in this department!.
With 2x gtx460s and a 2500k at 4ghz I was getting anywhere from 16 to 45 FPS @ 1080p on ultra. I have just grabbed a 7950 which should be arriving tomorrow but I would say performance is pretty solid, to max out the game you would need a gtx670 or 7950 higher (this is my prediction, I shall find out tomorrow!) but for atleast medium your system should handle it, probably high since the 480 has more vram

, the cpu might hold you back though, have you played Arma2 before? Because I found performance on my gtx460 about the same on a2 as a3 alpha, however the graphic improvements are insane so it is pretty efficient atm and should get even better before launch.
Oh a gtx580 should also be on high I would expect.
Gun sounds are excellent, I would again say they are better than BF3 (which I love!) they have so much punch in their sounds that it really is perfect. Not sure on destructible environments but they have made a big effort on physics this time round so I would expect a fair amount of features to be in the full release such as destructible environments however the sheer scale may mean we get much less than in bf3.
Not sure about steam but it does use their ingame browser which works

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