Arma's great but it's in no way the best looking shooter on PC.
Frankly I'm more impressed with this: http://www.youtube.com/watch?v=3WqNMgdzzjU
If you want to see a great-looking PC shooter you have to go all the way back to 2007's Crysis. The mods being created for it are STILL blowing minds. I saw a great demo last night for it but can't find it, so here's a random video.
Here's another one for all of us PS3 players to salivate over: http://imgur.com/gallery/CT8ZX