Arma II actually recommends a quad-core for optimal performance, it could be an indication that the industry is moving to support more than 2 cores effectively. Getting a quad-core is still more future-proofing than being practical when it comes to games IMHO. It won't hurt, but it's not strictly necessary (yet).
I7 is powerfull, the most powerfull core on the market, but it's not designed with gaming in mind (though it is indeed faster than Core 2). Personally I have 0 interest, but that has a lot to do with the fact I would have to get a new MB with a different socket to fit the CPU. And there lies IMO the main concern: get an older board, and you won't be able to upgrade in the future, since the new CPUs don't fit those boards. Another thing is that the Core 2 quad-cores aren't true quad-cores, they're 2 dual-cores stuck together. I7 quad-cores are "true" quads. I7 also has the memory controller on the CPU instead of in the MB northbride (AMD has been doing this for years btw).
If you don't feel like going I7, go for Phenom II. If you don't want AMD, go for I7 anyway so you can upgrade in the future. Better than buying an entirely new system... again. (something you'll be doing anyway if you go with Intel, but that's a different story)
Why is building your own not an option?
