ARMA II Looks beautiful, performance is based on how you wanna play it, if you want the game to look great with all the bells and whistles, you're gonna need an Geforce 200 series G-card or and SLI setup. Otherwise be prepared to have the graphics seriously pulled back. I have a Core 2 Duo 2.4ghz CPU, 6GB DDR800 Ram and two 8800GT's in SLI and It runs O.K, far from impressive though.
The game still packs that great simulation experience, Sounds are great, distant gun-battles and jets or helicopters sound life-like, especially when they fly over you, even the odd sand fly buzzing around your head can make you wanna swipe at thin air =). Gun physics are very real as are the gun models, not to mention the cockpits of every vehicle modelled down to every fine detail.
ARMA II does have some let-downs unfortunately, The User Interface lacks improvement, but overall co-ordination and ease of use, in-fact I'd say it's even worse than the all original Operation Flashpoint now, because you have to memorize every command menu on your keyboard, and that just doesn't work especially when your in a battle floundering just to get your men to go prone or engage the enemy.
Another bad point is the AI, this I found to be a major let down and caused immense frustration as they would struggle with even the simplest of commands, Mounting a vehicle or Helicopter for instance proves to be a nightmare, they either don't react, take a detour around the forest before mounting the vehicle which was right beside them in the first place, or they just get stuck on something. I often find myself leaving most of my team behind and doing missions with just one AI or even none, it defeats the purpose of the simulation but it's far less frustrating.
Overall I think the game still holds that authentic feeling to it, but is lacking a lot of much needed improvement.