Wel, although the i7 would be a great setup, as far as price/performance goes, honestly .. I'd reccomend a Phenom II chip. The 720 is great and very OC'able .. and then there are the great 955 and 965 Phenom II x4's which are very, very good when it comes to gaming performance. So yea if you just have a lot of extra money, the i7 920 is a win but considering the triple channel DDR3 RAM and 1366 mobo's are fairly costly, you're going to spend a good bit more than you would for some good DDR3 1333/a 790GX mobo/Phenom II core. So it basically depends on your budget. Either way is fine though. A Phenom II 955 and a good video card, coupled with good RAM would be more than enough power for ARMA 2 and pretty much whatever else your'e going to play in the near future.
I run the 955 and will soon have a Radeon 5870 and I don't think ANY games will give me any trouble even maxed for a hot minute. So don't get the idea AMD is any less of an option! ^_^